Medical School · Year 3 · Internal Medicine · includes a discussion video

Seminar 16: Anemia

Internal Medicine Clerkship

Learning Objectives

By the end of this seminar, students will be able to:

1. Classify anemias systematically using mean corpuscular volume and reticulocyte count to guide the differential diagnosis
2. Evaluate iron deficiency anemia through targeted history, appropriate laboratory testing, and identification of underlying etiology
3. Differentiate megaloblastic from non-megaloblastic causes of macrocytic anemia using clinical and laboratory features
4. Recognize the hallmark laboratory and clinical findings of hemolytic anemia and distinguish intrinsic from extrinsic causes
5. Apply evidence-based treatment strategies for specific anemia subtypes including iron replacement, vitamin supplementation, and erythropoiesis-stimulating agents
6. Describe transfusion indications, thresholds, reaction types, and management of transfusion-related complications

Seminar Outline

Section 1: Definition and Classification of Anemia

Anemia is defined as a reduction in hemoglobin concentration below established thresholds that vary by sex and physiologic state. In adult men, anemia is diagnosed when hemoglobin falls below 13 g/dL, while in non-pregnant adult women the threshold is 12 g/dL. During pregnancy, physiologic hemodilution lowers the diagnostic threshold to 11 g/dL. These definitions, established by the World Health Organization, provide a standardized framework for diagnosis, though clinicians must recognize that individual baseline values may differ based on altitude, ethnicity, and other demographic factors.

The most clinically useful initial classification of anemia relies on the mean corpuscular volume, which divides anemias into three broad categories. Microcytic anemias, defined by an MCV below 80 fL, include iron deficiency anemia and thalassemia as the most common etiologies. Normocytic anemias, with an MCV between 80 and 100 fL, encompass anemia of chronic disease and hemolytic anemias. Macrocytic anemias, characterized by an MCV exceeding 100 fL, are most often caused by vitamin B12 or folate deficiency and myelodysplastic syndromes. This MCV-based approach allows the clinician to rapidly narrow the differential diagnosis at the point of initial evaluation.

An alternative and complementary classification organizes anemias by their underlying pathophysiologic mechanism. Decreased red blood cell production accounts for anemias caused by nutritional deficiencies such as iron, B12, and folate, as well as aplastic anemia and bone marrow infiltration. Increased red blood cell destruction defines the hemolytic anemias, which may be intrinsic or extrinsic to the red blood cell membrane. Blood loss, whether acute from gastrointestinal hemorrhage or chronic from menstruation, constitutes the third mechanistic category. Understanding the mechanism is essential because it directly informs the therapeutic approach.

The reticulocyte count serves as a critical tool for distinguishing between hypoproliferative anemias and those caused by peripheral destruction or blood loss. A reticulocyte percentage below 2% indicates an inadequate bone marrow response, pointing toward a production defect such as nutritional deficiency or marrow failure. A reticulocyte percentage exceeding 2% suggests that the bone marrow is responding appropriately, directing the workup toward blood loss or hemolysis. The reticulocyte production index, calculated by multiplying the reticulocyte percentage by the patient's hematocrit divided by 45 and then dividing by a maturation factor, provides a more accurate assessment of marrow response, with a value greater than 2 indicating adequate erythropoietic activity.

<image>Panel A: Flowchart showing MCV-based classification of anemia into microcytic, normocytic, and macrocytic categories with representative etiologies listed under each. Panel B: Diagram illustrating the three mechanistic categories of anemia including decreased production, increased destruction, and blood loss with arrows indicating common causes. Panel C: Reticulocyte count interpretation algorithm showing the branch point at 2% separating hypoproliferative from hyperproliferative anemias. Panel D: Composite table showing reticulocyte index calculation with maturation correction factors at different hematocrit levels.</image>

Section 2: Iron Deficiency Anemia

Iron deficiency anemia is the most common cause of anemia worldwide and in clinical practice represents a critical diagnostic consideration because it frequently signals an underlying condition requiring investigation. The causes of iron deficiency can be organized into four categories: blood loss, decreased dietary intake, impaired absorption, and increased physiologic demand. In adult men and postmenopausal women, gastrointestinal blood loss is the most common etiology and mandates endoscopic evaluation to exclude malignancy. In premenopausal women, menstrual blood loss is the leading cause, though gastrointestinal sources should still be considered when clinical suspicion warrants. Malabsorptive conditions including celiac disease, prior gastric bypass surgery, and Helicobacter pylori infection can impair duodenal iron absorption, while pregnancy and periods of rapid growth increase iron requirements beyond dietary supply.

The diagnosis of iron deficiency anemia relies on a panel of iron studies that must be interpreted together rather than in isolation. Serum ferritin is the single most useful test, with a value below 30 ng/mL being highly specific for iron deficiency. Serum iron is characteristically low, while total iron-binding capacity is elevated, reflecting the liver's compensatory increase in transferrin production. Transferrin saturation, calculated as serum iron divided by TIBC, falls below 20% in iron deficiency. Distinguishing iron deficiency from anemia of chronic disease is a common clinical challenge; in chronic disease, ferritin is typically normal or elevated due to the acute phase response, while TIBC is low or normal rather than elevated. In ambiguous cases, a soluble transferrin receptor level or reticulocyte hemoglobin content can help differentiate the two conditions.

The clinical features of iron deficiency anemia reflect both the reduced oxygen-carrying capacity of the blood and tissue-specific effects of iron depletion. Fatigue and weakness are nearly universal symptoms, accompanied by exertional dyspnea as the anemia worsens. Physical examination may reveal conjunctival and palmar pallor. Distinctive findings specific to iron deficiency include pica, the craving for non-food substances such as ice or clay, as well as koilonychia, which refers to spoon-shaped nails resulting from abnormal nail matrix keratinization. Glossitis, presenting as a smooth and painful tongue, and angular cheilitis, manifesting as fissures at the corners of the mouth, are additional mucocutaneous findings that should prompt consideration of iron deficiency even before laboratory confirmation.

Treatment of iron deficiency anemia involves both correction of the deficiency and investigation and management of the underlying cause. Oral ferrous sulfate at a dose of 325 mg three times daily is the standard first-line therapy, providing approximately 65 mg of elemental iron per dose. Absorption is enhanced by taking iron on an empty stomach with vitamin C, though gastrointestinal side effects may necessitate dose reduction or alternate-day dosing. Intravenous iron formulations such as iron sucrose and ferric carboxymaltose are indicated when oral iron is not tolerated, when malabsorption is present, or when severe anemia requires rapid repletion. The reticulocyte count typically peaks at 7 to 10 days after initiating therapy, and hemoglobin should begin rising within 1 to 2 weeks, with oral iron continued for 3 to 6 months after hemoglobin normalization to replenish iron stores.

<image>Panel A: Diagram showing the four major categories of iron deficiency causes including blood loss, decreased intake, impaired absorption, and increased demand with clinical examples for each. Panel B: Comparative laboratory profile showing iron studies in iron deficiency versus anemia of chronic disease with serum iron, TIBC, transferrin saturation, and ferritin values. Panel C: Clinical photographs illustrating koilonychia, angular cheilitis, and glossitis as physical findings of iron deficiency. Panel D: Treatment algorithm showing the decision pathway from oral iron therapy through intravenous iron with expected timeline of reticulocyte and hemoglobin response.</image>

Section 3: Other Microcytic Anemias

The thalassemias represent a group of inherited disorders characterized by reduced synthesis of one or more globin chains, leading to ineffective erythropoiesis and microcytic anemia. Alpha-thalassemia results from gene deletions on chromosome 16 and ranges in severity from the silent carrier state with one gene deletion to hydrops fetalis with four gene deletions. Beta-thalassemia is caused by point mutations affecting the beta-globin gene and is classified as trait, intermedia, or major depending on the number and severity of mutations. Beta-thalassemia trait produces a mild microcytic anemia with characteristic target cells on peripheral smear, while beta-thalassemia major causes transfusion-dependent severe anemia presenting in childhood. Hemoglobin electrophoresis is the diagnostic test of choice, showing elevated hemoglobin A2 in beta-thalassemia trait and abnormal hemoglobin patterns in more severe forms.

Distinguishing thalassemia trait from iron deficiency anemia is a frequently tested clinical scenario because both conditions produce microcytic anemia. Several features help differentiate the two. In thalassemia trait, the red blood cell count is typically normal or elevated, whereas it is low in iron deficiency. The red cell distribution width is normal in thalassemia but elevated in iron deficiency, reflecting the greater variability in red cell size seen with iron depletion. The Mentzer index, calculated as MCV divided by the red blood cell count, yields a value below 13 in thalassemia and above 13 in iron deficiency. Most importantly, iron studies are normal in thalassemia trait, providing definitive differentiation when laboratory overlap creates diagnostic uncertainty.

Anemia of chronic disease, also termed anemia of chronic inflammation, is a normocytic or mildly microcytic anemia driven by the central regulatory protein hepcidin. In the setting of chronic infection, malignancy, or autoimmune disease, inflammatory cytokines stimulate hepatic production of hepcidin, which blocks the ferroportin channel on enterocytes and macrophages, effectively sequestering iron within cells and reducing its availability for erythropoiesis. The iron studies in anemia of chronic disease characteristically show low serum iron with a normal-to-elevated ferritin and a low-to-normal TIBC, a pattern that reflects iron sequestration rather than true iron deficiency. Treatment is directed at the underlying inflammatory condition, as iron supplementation alone does not correct the fundamental pathophysiologic disturbance.

Sideroblastic anemia is a microcytic anemia caused by defective heme synthesis within erythroid precursors, leading to the accumulation of iron-laden mitochondria that form a ring around the nucleus of developing red blood cells. The diagnosis is established by the presence of ring sideroblasts on bone marrow biopsy with Prussian blue staining. Causes include hereditary forms related to mutations in the ALAS2 gene and acquired forms associated with myelodysplastic syndromes, chronic alcohol use, lead poisoning, and certain medications such as isoniazid. Hereditary sideroblastic anemia may respond to pyridoxine supplementation, as vitamin B6 serves as a cofactor for the ALAS enzyme, while acquired forms often require supportive care with transfusions and iron chelation therapy to prevent secondary iron overload.

<image>Panel A: Genetic diagram showing alpha-thalassemia gene deletion patterns from silent carrier through hydrops fetalis with corresponding clinical severity. Panel B: Side-by-side comparison of thalassemia trait versus iron deficiency showing RBC count, RDW, Mentzer index, and iron studies. Panel C: Schematic of hepcidin-mediated iron sequestration in anemia of chronic disease showing blocked ferroportin channels on macrophages and enterocytes. Panel D: Bone marrow photomicrograph showing ring sideroblasts with Prussian blue staining alongside a diagram of defective heme synthesis in the mitochondria.</image>

Section 4: Macrocytic Anemia

Macrocytic anemias are divided into megaloblastic and non-megaloblastic categories based on the underlying pathophysiology, a distinction with important diagnostic and therapeutic implications. Megaloblastic anemias are caused by impaired DNA synthesis, most commonly due to deficiency of vitamin B12 or folate, both of which are essential cofactors in the thymidylate synthase pathway required for nucleotide production. The hallmark of megaloblastic anemia on peripheral blood smear is the presence of hypersegmented neutrophils, defined as neutrophils with five or more nuclear lobes, along with macro-ovalocytes. These findings reflect the nuclear-cytoplasmic dyssynchrony that results from delayed nuclear maturation in the setting of normal cytoplasmic development, a phenomenon that affects all rapidly dividing cell lines.

Vitamin B12 deficiency deserves special attention because of its unique neurologic complications and the variety of conditions that can cause it. Pernicious anemia, an autoimmune condition characterized by antibodies against intrinsic factor and parietal cells, is the most common cause of B12 deficiency in developed countries. Other causes include gastrectomy and gastric bypass surgery, which eliminate the site of intrinsic factor production, and diseases of the terminal ileum such as Crohn disease, which impair B12 absorption. Strict vegan diets lacking all animal products can lead to B12 deficiency over several years because body stores are substantial. Medications including metformin and proton pump inhibitors may also contribute to B12 depletion through impaired absorption.

Differentiating B12 from folate deficiency is clinically essential because B12 deficiency produces neurologic manifestations that folate deficiency does not. Subacute combined degeneration of the spinal cord, the classic neurologic complication of B12 deficiency, involves demyelination of the dorsal columns and lateral corticospinal tracts, producing loss of proprioception and vibratory sense along with upper motor neuron signs. Both deficiencies cause elevated serum homocysteine levels, but only B12 deficiency produces elevated methylmalonic acid, making MMA the key distinguishing laboratory test. B12 stores last for years while folate stores are depleted within months, explaining the different timelines of deficiency development. Critically, if both deficiencies coexist, B12 must be repleted first because folate supplementation alone can correct the hematologic abnormalities while allowing irreversible neurologic damage to progress.

Non-megaloblastic macrocytic anemias encompass a heterogeneous group of conditions that produce macrocytosis through mechanisms other than impaired DNA synthesis. Chronic alcohol use is one of the most common causes, producing macrocytosis through direct toxic effects on erythroid precursors independent of folate deficiency. Liver disease causes macrocytosis through alterations in the lipid composition of the red blood cell membrane, leading to an increased surface area. Hypothyroidism produces macrocytosis through unclear mechanisms and should be screened for with a TSH level. Myelodysplastic syndromes represent a clonal bone marrow disorder that frequently presents with macrocytic anemia and dysplastic changes on smear. Reticulocytosis from any cause can produce a spuriously elevated MCV because reticulocytes are larger than mature erythrocytes, and medications such as hydroxyurea and zidovudine directly interfere with DNA synthesis to cause macrocytosis.

<image>Panel A: Peripheral blood smear comparison showing hypersegmented neutrophils and macro-ovalocytes in megaloblastic anemia versus round macrocytes in non-megaloblastic macrocytosis. Panel B: Anatomical diagram showing sites of B12 absorption and causes of deficiency including pernicious anemia, gastrectomy, and terminal ileum disease. Panel C: Diagnostic flowchart differentiating B12 from folate deficiency using homocysteine and methylmalonic acid levels. Panel D: Summary chart of non-megaloblastic macrocytic anemia causes including alcohol, liver disease, hypothyroidism, MDS, reticulocytosis, and medications with their mechanisms.</image>

Section 5: Hemolytic Anemia

Hemolytic anemias are characterized by premature destruction of red blood cells and are broadly classified as intrinsic or extrinsic based on whether the defect resides within the red blood cell itself or is imposed by external factors. Intrinsic hemolytic anemias include disorders of the red cell membrane such as hereditary spherocytosis, enzyme deficiencies such as glucose-6-phosphate dehydrogenase deficiency, and hemoglobin disorders such as sickle cell disease. Extrinsic hemolytic anemias include autoimmune hemolytic anemia, thrombotic thrombocytopenic purpura, and mechanical hemolysis from prosthetic heart valves. A further classification divides hemolysis into intravascular, occurring within blood vessels, and extravascular, occurring within the reticuloendothelial system particularly the spleen. This distinction affects the pattern of laboratory abnormalities and guides the diagnostic evaluation.

The laboratory evaluation of suspected hemolysis follows a predictable pattern reflecting the consequences of accelerated red blood cell destruction. The reticulocyte count is elevated, indicating an appropriate bone marrow response to increased peripheral red cell loss. Lactate dehydrogenase is elevated because it is released from lysed red blood cells. Indirect bilirubin rises as the heme degradation pathway is overwhelmed by the increased bilirubin load. Haptoglobin, which binds free hemoglobin released during hemolysis, is characteristically low or undetectable as it is consumed faster than it can be produced. The direct antiglobulin test, also known as the Coombs test, detects antibodies or complement bound to the red cell surface and is the key test for distinguishing autoimmune hemolytic anemia from other causes. Peripheral blood smear examination reveals morphologic clues including spherocytes in autoimmune hemolysis and hereditary spherocytosis, and schistocytes in microangiopathic hemolytic anemia.

Autoimmune hemolytic anemia is classified by the thermal reactivity of the offending antibody. Warm autoimmune hemolytic anemia, mediated by IgG antibodies that bind red blood cells optimally at body temperature, is the more common type and is associated with autoimmune diseases, lymphoproliferative disorders, and medications such as methyldopa. The IgG-coated red blood cells are partially phagocytosed in the spleen, producing spherocytes that are subsequently trapped and destroyed in further splenic passages. Cold autoimmune hemolytic anemia is mediated by IgM antibodies that bind red blood cells at temperatures below 37 degrees Celsius, fixing complement and causing intravascular hemolysis. Cold agglutinin disease is associated with Mycoplasma pneumoniae infections, Epstein-Barr virus, and lymphoproliferative disorders. Treatment of warm AIHA centers on corticosteroids as first-line therapy, while cold AIHA is managed primarily by cold avoidance and treatment of any underlying condition.

Microangiopathic hemolytic anemia represents a category of hemolysis caused by mechanical shearing of red blood cells as they traverse damaged or obstructed microvasculature, producing the characteristic schistocytes on peripheral smear. Thrombotic thrombocytopenic purpura, caused by severe deficiency of the ADAMTS13 metalloprotease, presents with the classic pentad of microangiopathic hemolytic anemia, thrombocytopenia, neurologic symptoms, renal dysfunction, and fever, though the full pentad is present in only a minority of cases. Hemolytic uremic syndrome, most commonly triggered by Shiga toxin-producing Escherichia coli, presents with hemolytic anemia, thrombocytopenia, and acute renal failure, predominantly affecting children. Disseminated intravascular coagulation produces a consumptive coagulopathy with microangiopathic hemolysis in the setting of sepsis, malignancy, obstetric emergencies, or massive trauma. Recognition of schistocytes on the peripheral smear is the critical diagnostic clue that should prompt immediate further evaluation.

<image>Panel A: Classification tree of hemolytic anemias dividing intrinsic from extrinsic causes and intravascular from extravascular hemolysis with representative conditions. Panel B: Laboratory panel showing the characteristic findings of hemolysis including elevated reticulocytes, LDH, and indirect bilirubin with low haptoglobin and positive DAT. Panel C: Diagram comparing warm IgG-mediated and cold IgM-mediated autoimmune hemolytic anemia including temperature of reactivity, splenic versus intravascular destruction, and associated conditions. Panel D: Peripheral smear images showing schistocytes in microangiopathic hemolytic anemia with comparison of TTP, HUS, and DIC clinical features.</image>

Section 6: Specific Hemolytic Disorders

Sickle cell disease is an autosomal recessive hemoglobinopathy caused by a point mutation in the beta-globin gene that produces hemoglobin S, which polymerizes under conditions of deoxygenation, causing the characteristic sickle-shaped deformation of red blood cells. The homozygous genotype HbSS produces sickle cell disease, while heterozygotes with HbAS have sickle cell trait, which is generally asymptomatic except under extreme physiologic stress. The clinical manifestations of sickle cell disease result from vaso-occlusion by sickled red blood cells, chronic hemolysis, and progressive organ damage. Vaso-occlusive crises produce severe pain episodes affecting the bones, chest, and abdomen. Aplastic crises, triggered by parvovirus B19 infection, cause a sudden cessation of erythropoiesis, while splenic sequestration crises involve the acute pooling of blood in the spleen. Long-term complications include stroke, acute chest syndrome, priapism, avascular necrosis, and progressive renal disease. Treatment involves hydroxyurea as the cornerstone disease-modifying therapy, chronic transfusions for stroke prevention, and aggressive pain management during crises.

Glucose-6-phosphate dehydrogenase deficiency is the most common human enzyme deficiency worldwide, affecting over 400 million people, and is inherited in an X-linked recessive pattern, predominantly affecting males. G6PD catalyzes the first step of the pentose phosphate pathway, which generates NADPH required to maintain glutathione in its reduced form, protecting red blood cells from oxidative damage. When exposed to oxidant stressors such as fava beans, sulfonamides, dapsone, primaquine, or nitrofurantoin, affected individuals develop episodic intravascular hemolysis because their red blood cells cannot neutralize reactive oxygen species. The peripheral smear during an acute hemolytic episode shows bite cells, which result from splenic removal of Heinz bodies, which are precipitates of denatured hemoglobin visible on supravital staining. The G6PD enzyme level may be falsely normal during acute hemolysis because the most deficient older red blood cells have already been destroyed, necessitating repeat testing after recovery. Treatment is primarily preventive, involving avoidance of known oxidant triggers.

Hereditary spherocytosis is the most common inherited red blood cell membrane disorder, resulting from defects in structural proteins including spectrin, ankyrin, band 3, and protein 4.2 that maintain the normal biconcave disc shape. Loss of membrane surface area relative to cell volume produces spherocytes, which are rigid, sphere-shaped cells that are preferentially trapped and destroyed during passage through the narrow sinusoidal spaces of the spleen, resulting in extravascular hemolysis. The peripheral smear shows spherocytes, which appear as small, densely staining red blood cells lacking the normal central pallor. Diagnosis is supported by the osmotic fragility test, in which spherocytes lyse at higher osmotic concentrations than normal red blood cells, and by the eosin-5-maleimide binding test, which quantifies band 3 protein on the red cell surface using flow cytometry. Splenectomy is the definitive treatment for patients with severe hemolysis, eliminating the primary site of red blood cell destruction, though it is typically deferred until after age 5 to minimize the risk of overwhelming post-splenectomy infection.

Paroxysmal nocturnal hemoglobinuria is a rare acquired clonal disorder of hematopoietic stem cells caused by a somatic mutation in the PIGA gene, which is required for synthesis of the glycosylphosphatidylinositol anchor that attaches complement regulatory proteins to the cell surface. The loss of CD55 and CD59, two GPI-anchored complement inhibitors, renders affected red blood cells exquisitely sensitive to complement-mediated lysis, producing chronic intravascular hemolysis that classically worsens at night due to mild respiratory acidosis during sleep. The clinical triad of PNH includes hemolytic anemia, thrombosis at unusual sites such as the hepatic and cerebral veins, and bone marrow failure manifesting as pancytopenia. Diagnosis is established by flow cytometry demonstrating the absence of GPI-anchored proteins CD55 and CD59 on red blood cells and white blood cells. Eculizumab, a monoclonal antibody that inhibits the complement protein C5, has revolutionized the treatment of PNH by preventing formation of the membrane attack complex and dramatically reducing hemolysis, thrombotic events, and transfusion requirements.

<image>Panel A: Molecular diagram showing hemoglobin S polymerization under deoxygenation with progression from normal discoid to sickled red blood cell shape and vaso-occlusion in capillaries. Panel B: Pathway diagram of G6PD function in the pentose phosphate pathway showing NADPH generation, glutathione recycling, and oxidative stress leading to Heinz body formation and bite cells. Panel C: Structural diagram of the red cell membrane showing spectrin, ankyrin, and band 3 defects in hereditary spherocytosis with resulting loss of membrane and formation of spherocytes. Panel D: Illustration of the GPI anchor deficiency in PNH showing absent CD55 and CD59 on the red cell surface with complement-mediated lysis and flow cytometry diagnostic pattern.</image>

Section 7: Normocytic Anemia

The approach to normocytic anemia begins with the reticulocyte count, which serves as the fundamental branch point in the diagnostic algorithm. A low reticulocyte count in the setting of normocytic anemia indicates a hypoproliferative state in which the bone marrow is failing to produce red blood cells at an adequate rate to compensate for normal turnover or increased loss. An elevated reticulocyte count suggests that the marrow is responding appropriately to peripheral red blood cell destruction or blood loss, directing the evaluation toward hemolytic and hemorrhagic etiologies. Review of the peripheral blood smear provides additional morphologic clues, including rouleaux formation in multiple myeloma, target cells in liver disease, and teardrop cells in myelofibrosis. Integration of the reticulocyte count with the smear findings allows for a focused and efficient diagnostic workup.

Anemia of chronic kidney disease represents one of the most common causes of normocytic, hypoproliferative anemia encountered in clinical practice. The pathophysiology centers on decreased production of erythropoietin by the diseased kidneys, compounded by the suppressive effects of uremic toxins on erythroid progenitor cells in the bone marrow. Anemia typically becomes clinically significant when the estimated glomerular filtration rate falls below 30 mL/min/1.73m2, corresponding to stage 4 chronic kidney disease. Treatment involves erythropoiesis-stimulating agents such as epoetin alfa and darbepoetin alfa, with a target hemoglobin of 10 to 11.5 g/dL, as higher targets have been associated with increased cardiovascular events and mortality in clinical trials. Iron supplementation is frequently required as a co-intervention because functional iron deficiency is common in CKD patients, and adequate iron stores are a prerequisite for an optimal response to ESA therapy.

Bone marrow failure syndromes represent a diverse group of disorders that cause normocytic anemia through impaired hematopoiesis. Aplastic anemia is characterized by pancytopenia with a hypocellular bone marrow, most commonly resulting from immune-mediated destruction of hematopoietic stem cells, though toxic exposures and inherited conditions such as Fanconi anemia may also be causative. Myelodysplastic syndromes are clonal bone marrow disorders defined by cytopenias and morphologic dysplasia, typically occurring in older adults and carrying a variable risk of transformation to acute myeloid leukemia. Myelofibrosis produces normocytic anemia along with teardrop-shaped red blood cells and splenomegaly due to progressive bone marrow fibrosis and extramedullary hematopoiesis. Acute leukemia may present with normocytic anemia accompanied by circulating blast cells on the peripheral smear, representing a hematologic emergency requiring immediate evaluation.

The systematic workup of unexplained normocytic anemia involves a stepwise evaluation to exclude the most common and treatable causes before proceeding to bone marrow biopsy. The reticulocyte count is the essential first step, distinguishing production defects from destructive processes. Iron studies including ferritin and transferrin saturation should be obtained to rule out iron deficiency, which can occasionally present with a normal MCV particularly when coexisting conditions such as B12 deficiency or liver disease independently raise the MCV. Serum B12 and folate levels exclude nutritional deficiencies that may present atypically. Lactate dehydrogenase and haptoglobin levels screen for occult hemolysis. When the initial evaluation does not reveal a clear etiology, bone marrow biopsy with aspirate is indicated to evaluate for aplastic anemia, myelodysplastic syndromes, infiltrative processes, and other primary bone marrow pathology.

<image>Panel A: Diagnostic algorithm for normocytic anemia beginning with reticulocyte count and branching into hypoproliferative and hyperproliferative pathways with subsequent testing steps. Panel B: Diagram of erythropoietin production in chronic kidney disease showing decreased renal synthesis, uremic suppression of erythroid precursors, and the mechanism of ESA therapy. Panel C: Bone marrow biopsy comparison showing normal cellularity, hypocellular marrow in aplastic anemia, dysplastic changes in MDS, and fibrosis in myelofibrosis. Panel D: Stepwise workup chart for unexplained normocytic anemia showing iron studies, B12/folate, hemolysis labs, and bone marrow biopsy with expected findings at each step.</image>

Section 8: Treatment Principles

Iron replacement therapy is the cornerstone of treatment for iron deficiency anemia and can be administered through oral or intravenous routes depending on clinical circumstances. Oral iron in the form of ferrous sulfate is the standard first-line therapy for most patients, providing cost-effective and readily available supplementation. The classic dose of ferrous sulfate 325 mg three times daily delivers approximately 195 mg of elemental iron per day, though recent evidence suggests that alternate-day dosing may improve fractional absorption due to the transient hepcidin surge that follows each oral iron dose. Intravenous iron is indicated when oral supplementation fails due to malabsorption, medication intolerance, chronic kidney disease requiring erythropoiesis-stimulating agents, or when the severity of anemia demands rapid correction. Common intravenous formulations include iron sucrose, ferric carboxymaltose, and ferric derisomaltose, with ferric carboxymaltose offering the advantage of complete repletion in as few as one to two infusions.

Vitamin B12 and folate replacement follow distinct protocols dictated by the underlying cause and the risk of neurologic complications. B12 replacement can be administered intramuscularly, typically as cyanocobalamin 1000 mcg daily for one week, then weekly for one month, then monthly for life in cases of pernicious anemia or permanent malabsorption. High-dose oral B12 at 1000 to 2000 mcg daily has been shown to be effective even in patients with pernicious anemia, as approximately 1% of an oral dose is absorbed through passive diffusion independent of intrinsic factor. Folate is replaced orally at 1 to 5 mg daily and is continued until the underlying cause is corrected. A critical clinical principle is that when both deficiencies coexist, B12 must be replaced first, because folate supplementation can mask the hematologic manifestations of B12 deficiency while allowing the potentially irreversible neurologic damage of subacute combined degeneration to progress unchecked.

Erythropoiesis-stimulating agents are primarily used in the management of anemia associated with chronic kidney disease and chemotherapy-induced anemia in cancer patients. Epoetin alfa requires more frequent dosing, typically two to three times weekly for CKD patients, while darbepoetin alfa has a longer half-life permitting weekly or biweekly administration. The target hemoglobin for ESA therapy is 10 to 11.5 g/dL, as randomized controlled trials including CHOIR and TREAT demonstrated that targeting higher hemoglobin levels was associated with increased risks of hypertension, thromboembolic events, cardiovascular mortality, and possible tumor progression. Iron status must be optimized before and during ESA therapy, as functional iron deficiency is the most common cause of suboptimal ESA response. Other causes of ESA resistance include chronic inflammation, infection, hyperparathyroidism, and aluminum toxicity.

Transfusion decisions in anemia management are guided by a combination of hemoglobin thresholds and clinical symptomatology rather than by laboratory values alone. For hemodynamically stable patients with chronic anemia, a restrictive transfusion threshold of hemoglobin below 7 g/dL is supported by evidence from the TRICC trial and subsequent studies demonstrating equivalent or superior outcomes compared to liberal transfusion strategies. Patients with acute coronary syndromes may benefit from a slightly higher threshold of 8 g/dL, though the optimal target remains an area of active investigation. In acute hemorrhage, transfusion decisions are guided by the clinical assessment of blood loss volume, hemodynamic stability, and ongoing bleeding rather than by a specific hemoglobin cutoff. The general practice is to transfuse one to two units of packed red blood cells and then reassess rather than ordering multiple units empirically, as each unit of packed red blood cells is expected to raise the hemoglobin by approximately 1 g/dL in a non-bleeding patient.

<image>Panel A: Comparison of oral versus intravenous iron replacement showing indications, common formulations, dosing schedules, and expected response timelines. Panel B: B12 replacement protocol diagram showing intramuscular and high-dose oral routes with dosing schedules for pernicious anemia and dietary deficiency. Panel C: Chart of erythropoiesis-stimulating agent therapy showing target hemoglobin range, dosing of epoetin versus darbepoetin, and causes of ESA resistance. Panel D: Transfusion threshold algorithm showing restrictive versus liberal strategies with hemoglobin targets for chronic anemia, coronary artery disease, and acute hemorrhage.</image>

Section 9: Blood Transfusion

Red blood cell transfusion is a life-saving intervention that requires meticulous attention to compatibility testing and clinical monitoring to minimize the risk of adverse events. The primary indication for transfusion is symptomatic anemia or hemoglobin below 7 g/dL in most clinical contexts, though the threshold may be adjusted based on patient comorbidities and clinical trajectory. Pre-transfusion testing begins with ABO and Rh typing, followed by an antibody screen to detect clinically significant alloantibodies, and concludes with a crossmatch to verify compatibility between the donor unit and the recipient's serum. Each unit of packed red blood cells has a volume of approximately 300 mL and is expected to increase the hemoglobin by approximately 1 g/dL and the hematocrit by 3% in a standard-sized adult who is not actively bleeding.

Transfusion reactions span a spectrum from mild allergic responses to life-threatening hemolytic emergencies, and clinicians must be able to recognize and manage each type promptly. Acute hemolytic transfusion reactions result from ABO incompatibility and present within minutes of transfusion onset with fever, rigors, flank pain, hemoglobinuria, and potentially disseminated intravascular coagulation and cardiovascular collapse. Febrile non-hemolytic transfusion reactions are caused by cytokines released from donor leukocytes and present with fever and chills without hemolysis. Allergic reactions range from mild urticaria to severe anaphylaxis, with the latter occurring most commonly in IgA-deficient recipients. Transfusion-related acute lung injury manifests as non-cardiogenic pulmonary edema within 2 to 6 hours of transfusion and is caused by donor antibodies that activate recipient neutrophils in the pulmonary vasculature. Transfusion-associated circulatory overload presents similarly to TRALI but results from volume overload rather than immune-mediated lung injury, particularly in patients with compromised cardiac function.

The management of transfusion reactions begins with the universal first step of immediately stopping the transfusion and maintaining intravenous access for any suspected reaction. For acute hemolytic reactions, aggressive fluid resuscitation to maintain renal perfusion is critical, along with monitoring for and treating DIC and sending the blood bank a post-reaction sample for direct antiglobulin testing and clerical check. Febrile non-hemolytic reactions are managed with antipyretics, and premedication with acetaminophen is recommended for future transfusions. Mild allergic reactions respond to antihistamines, while anaphylactic reactions require immediate epinephrine, airway management, and volume resuscitation. TRALI is managed with supportive care including supplemental oxygen and mechanical ventilation if needed, and the implicated donor is deferred from future donation. The blood bank must be notified of all transfusion reactions to facilitate investigation and prevent future occurrences.

Special blood products are available for patients with specific clinical needs that cannot be met by standard packed red blood cells. Leukoreduced blood products, from which white blood cells have been removed by filtration, reduce the incidence of febrile non-hemolytic transfusion reactions, CMV transmission, and HLA alloimmunization, and are now standard in many blood banks. Irradiated blood products have been exposed to gamma radiation to prevent transfusion-associated graft-versus-host disease by inactivating donor lymphocytes, and are indicated for immunocompromised patients including those undergoing hematopoietic stem cell transplantation, patients receiving purine analog chemotherapy, and recipients of directed donations from blood relatives. CMV-negative blood products are reserved for CMV-seronegative recipients at risk for primary CMV infection, particularly pregnant women and neonates. Washed red blood cells, from which residual plasma proteins have been removed, are indicated for patients with IgA deficiency who have anti-IgA antibodies and are at risk for anaphylactic transfusion reactions.

<image>Panel A: Flowchart of pre-transfusion testing showing ABO/Rh typing, antibody screen, and crossmatch steps with expected outcomes. Panel B: Timeline diagram of transfusion reactions showing acute hemolytic, febrile non-hemolytic, allergic, TRALI, and TACO with onset timing and distinguishing clinical features. Panel C: Emergency management algorithm for transfusion reactions beginning with stopping the transfusion and branching by reaction type with specific interventions. Panel D: Summary of special blood products including leukoreduced, irradiated, CMV-negative, and washed products with their indications and mechanisms of benefit.</image>

Section 10: Clinical Pearls and Special Populations

Anemia in the elderly represents a unique diagnostic challenge because it is frequently multifactorial and may lack a single identifiable cause despite thorough evaluation. Approximately one-third of anemia cases in patients over age 65 remain unexplained after standard workup, a condition termed unexplained anemia of the elderly, which may relate to age-related decline in hematopoietic stem cell function, subclinical inflammation, or decreased androgen and erythropoietin levels. The evaluation should be comprehensive and include iron studies, B12 and folate levels, renal function assessment, and consideration of myelodysplastic syndromes, which increase in incidence with advancing age. Even mild anemia in elderly patients is clinically significant, as it is independently associated with increased mortality, falls, cognitive decline, hospitalization, and reduced quality of life, emphasizing the importance of not dismissing mild anemia as a normal consequence of aging.

Anemia in pregnancy requires special consideration because of the physiologic changes in blood volume that occur during gestation. Plasma volume expands by approximately 50% during pregnancy while red blood cell mass increases by only 25%, producing a physiologic dilutional anemia that does not represent true red blood cell deficiency. Pathologic anemia is defined by hemoglobin below 11 g/dL in the first and third trimesters and below 10.5 g/dL in the second trimester, when hemodilution is maximal. Iron deficiency is the most common pathologic cause of anemia in pregnancy, driven by the increased iron demands of the expanding maternal blood volume, the developing fetus, and the placenta, which together require approximately 1000 mg of additional iron over the course of gestation. Treatment with oral or intravenous iron is safe and effective, and all pregnant women should receive routine screening for anemia at the initial prenatal visit and again in the third trimester.

Common clinical scenarios tested in medical education and encountered in practice can be efficiently approached using pattern recognition combined with targeted laboratory evaluation. A microcytic anemia with a low ferritin definitively establishes iron deficiency, while a microcytic anemia with a normal ferritin and target cells on smear suggests thalassemia trait. Macrocytic anemia accompanied by neurologic symptoms including paresthesias, gait ataxia, and cognitive changes should prompt immediate evaluation for B12 deficiency with serum B12 and methylmalonic acid levels. A normocytic anemia with an elevated reticulocyte count and low haptoglobin points to hemolysis, while a normocytic anemia with a low reticulocyte count in the setting of chronic kidney disease indicates renal anemia. Mastery of these pattern-based approaches allows for rapid diagnosis and timely initiation of appropriate therapy.

Red flags in the evaluation of anemia should prompt urgent investigation because they may indicate life-threatening underlying conditions. Pancytopenia, the simultaneous reduction of all three cell lines, raises concern for bone marrow failure from aplastic anemia, acute leukemia, or myelodysplastic syndrome and generally warrants bone marrow biopsy. The presence of blast cells on the peripheral blood smear is pathognomonic for acute leukemia and requires immediate hematology consultation. Schistocytes on the smear indicate microangiopathic hemolytic anemia, raising the possibility of TTP, HUS, or DIC, all of which are medical emergencies. A rapid decline in hemoglobin without obvious external bleeding should prompt evaluation for occult hemorrhage, particularly retroperitoneal or intra-abdominal sources, as well as intravascular hemolysis and splenic sequestration. These red flag findings should never be attributed to benign causes without a thorough and expedited workup.

<image>Panel A: Venn diagram showing the multifactorial causes of anemia in the elderly including nutritional deficiency, chronic disease, renal insufficiency, myelodysplasia, and unexplained anemia with their overlapping contributions. Panel B: Graph showing plasma volume and red blood cell mass expansion during pregnancy with the resultant physiologic anemia nadir in the second trimester. Panel C: Pattern recognition grid showing five classic anemia scenarios with their key laboratory findings and most likely diagnoses. Panel D: Red flag alert panel showing pancytopenia, blasts on smear, schistocytes, and rapid hemoglobin decline with the urgent conditions they suggest and recommended immediate actions.</image>

Summary

• Classify anemia by MCV: microcytic (below 80 fL), normocytic (80 to 100 fL), macrocytic (above 100 fL)
• Reticulocyte count distinguishes production defects (low) from destruction or blood loss (high)
• Iron deficiency anemia: low ferritin is most specific; investigate for GI blood loss in men and postmenopausal women
• B12 deficiency: neurologic symptoms distinguish it from folate deficiency; elevated MMA is confirmatory; treat before folate
• Hemolytic anemia: elevated LDH and indirect bilirubin, low haptoglobin, and elevated reticulocyte count
• Autoimmune hemolytic anemia: positive DAT; warm type (IgG) treated with steroids, cold type (IgM) managed with cold avoidance
• Sickle cell disease: vaso-occlusive crises are the hallmark; hydroxyurea is disease-modifying therapy
• CKD anemia: decreased erythropoietin production; treated with ESAs targeting hemoglobin 10 to 11.5 g/dL
• Transfuse for symptoms or hemoglobin below 7 g/dL in most settings; monitor for reactions
• Always consider multiple simultaneous causes of anemia, especially in elderly patients

Key Terms

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