# Anemia: A Diagnostic Framework for the Internist

## Overview

Anemia is defined by a hemoglobin below 13 g/dL in men and below 12 g/dL in women according to WHO criteria. It is one of the most common conditions encountered on the wards and in clinic, and it is always a symptom of an underlying process rather than a diagnosis in itself. The goal is to identify and treat the cause. A systematic approach combining MCV-based classification with the reticulocyte count provides an efficient diagnostic framework that guides the workup from the very first set of labs.

## Diagnostic Approach

### Step 1: Confirm Anemia and Assess Severity

Anemia severity is categorized as mild (hemoglobin 10-12 in women or 10-13 in men), moderate (hemoglobin 8-10), or severe (hemoglobin below 8). When evaluating a patient, assess for symptoms such as fatigue, dyspnea on exertion, tachycardia, pallor, angina, and signs of heart failure. The severity of symptoms often correlates more with the rapidity of onset than the absolute hemoglobin level.

### Step 2: Reticulocyte Count

The reticulocyte count reflects the bone marrow's response to anemia. An elevated reticulocyte index (greater than 2) indicates an appropriate marrow response, pointing toward blood loss or hemolysis as the underlying mechanism. A low or normal reticulocyte index (less than 2) indicates an inadequate marrow response, suggesting a hypoproliferative anemia caused by a production problem. The reticulocyte production index (RPI) provides a more accurate assessment and is calculated as the reticulocyte percentage multiplied by the patient's hematocrit divided by the normal hematocrit, then divided by a maturation factor that varies with the degree of anemia.

### Step 3: MCV-Based Classification

<image>MCV-based diagnostic algorithm for anemia showing microcytic (MCV < 80), normocytic (MCV 80-100), and macrocytic (MCV > 100) categories with key etiologies and initial laboratory workup for each</image>

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## Microcytic Anemia (MCV < 80 fL)

### Iron Deficiency Anemia (IDA) — Most Common Cause of Anemia Worldwide

| Iron Study | Iron Deficiency | Anemia of Chronic Disease | Both (IDA + ACD) |
|-----------|----------------|--------------------------|-----------------|
| Ferritin | <30 (diagnostic) | Normal or elevated | 30-100 |
| Serum iron | Low | Low | Low |
| TIBC | High | Low | Low-normal |
| Transferrin saturation | <20% | Low (10-20%) | <20% |
| Soluble transferrin receptor | Elevated | Normal | Elevated |

Iron deficiency anemia results from chronic blood loss (with GI sources being the most common, including occult colon cancer, menstruation, and peptic ulcer disease), poor dietary intake, malabsorption (celiac disease, bariatric surgery, PPI use), or increased demands such as pregnancy. The iron studies pattern is distinctive: ferritin below 30 ng/mL is diagnostic (below 15 is definitive), though ferritin between 30 and 100 may still represent IDA in the setting of inflammation since ferritin is an acute-phase reactant. Serum iron is low, TIBC is elevated, and transferrin saturation falls below 20%. The peripheral smear shows microcytic, hypochromic red blood cells along with target cells and pencil cells.

Management begins with identifying and addressing the source of blood loss. Colonoscopy and EGD are mandatory in men and postmenopausal women with unexplained IDA to exclude malignancy. For iron replacement, oral ferrous sulfate 325 mg (providing 65 mg of elemental iron) taken daily on an empty stomach with vitamin C is standard, and every-other-day dosing may actually improve absorption through hepcidin regulation. IV iron is preferred when oral iron fails, when malabsorption is present, in severe anemia, with ongoing blood loss, in CKD, or in inflammatory states. Options include iron sucrose, ferric carboxymaltose, and ferumoxytol.

### Thalassemia

Thalassemia is a genetic disorder of globin chain synthesis that leads to ineffective erythropoiesis. Alpha thalassemia involves gene deletions ranging from the silent carrier state to HbH disease to the fatal Hb Bart hydrops fetalis. Beta thalassemia is classified as minor (trait), intermedia, or major (transfusion-dependent). Diagnosis is made by hemoglobin electrophoresis, which shows elevated HbA2 and HbF in beta thalassemia trait. Key features distinguishing thalassemia from IDA include an elevated (not decreased) RBC count, a normal RDW (versus elevated in IDA), and a Mentzer index (MCV divided by RBC count) below 13. Iron studies are normal in thalassemia unless coexisting iron deficiency or iron overload from transfusions is present.

### Anemia of Chronic Disease/Inflammation (can be microcytic or normocytic)

This condition results from cytokine-mediated iron sequestration via hepcidin. The iron studies show low iron, low TIBC, normal or elevated ferritin, and low transferrin saturation. Treatment is directed at the underlying inflammatory condition, with erythropoiesis-stimulating agents considered if the anemia is CKD-associated.

### Sideroblastic Anemia

Sideroblastic anemia is characterized by ringed sideroblasts on bone marrow biopsy. Causes include myelodysplastic syndromes, lead poisoning, alcohol use, isoniazid (through pyridoxine deficiency), and copper deficiency.

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## Normocytic Anemia (MCV 80-100 fL)

### With Low Reticulocyte Count (Hypoproliferative)

The most common cause is anemia of chronic disease or inflammation. CKD-associated anemia results from decreased erythropoietin production. Anemia of acute blood loss appears normocytic early, before the reticulocyte response kicks in at 3 to 5 days. Bone marrow failure states include aplastic anemia, myelofibrosis, myelodysplastic syndromes, and infiltrative disease such as leukemia or metastatic cancer. Endocrine causes encompass hypothyroidism, adrenal insufficiency, and hypogonadism. Notably, a mixed nutritional deficiency combining iron with B12 or folate deficiency can produce a normal MCV, masking what would otherwise be a micro- or macrocytic picture.

### With High Reticulocyte Count (Hemolytic or Blood Loss)

An elevated reticulocyte count with normocytic anemia points toward hemolytic anemias or acute blood loss after the marrow has had 3 to 5 days to respond.

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## Macrocytic Anemia (MCV > 100 fL)

### Megaloblastic (Impaired DNA Synthesis)

Vitamin B12 deficiency has multiple causes including pernicious anemia (anti-intrinsic factor antibodies), malabsorption from bariatric surgery, Crohn disease, or celiac disease, dietary insufficiency in strict vegans, and medications such as metformin, PPIs, and H2 blockers. Laboratory findings include low B12 with elevated methylmalonic acid and homocysteine. Neurologic findings are particularly concerning and include subacute combined degeneration affecting the posterior columns and corticospinal tracts, peripheral neuropathy, and cognitive changes that may be irreversible if treatment is delayed. Treatment consists of cyanocobalamin 1000 mcg IM daily for 7 days, then weekly for 4 weeks, then monthly, or high-dose oral supplementation (1000-2000 mcg/day) if absorption is adequate.

Folate deficiency results from poor dietary intake, alcoholism, malabsorption, increased requirements (pregnancy, hemolytic anemia), or medications (methotrexate, trimethoprim, phenytoin). Labs show low folate with elevated homocysteine but normal methylmalonic acid. Treatment is folic acid 1 to 5 mg PO daily. Critically, B12 must always be checked before treating folate deficiency because folate supplementation can mask B12 deficiency and allow neurologic damage to progress unchecked.

### Non-Megaloblastic

Non-megaloblastic macrocytosis can result from alcoholism (direct marrow toxicity and nutritional deficiency), liver disease, hypothyroidism, myelodysplastic syndromes, medications (hydroxyurea, methotrexate, azathioprine, zidovudine), and reticulocytosis (since reticulocytes are larger than mature RBCs and can raise the MCV).

<image>Iron studies patterns comparing iron deficiency anemia, anemia of chronic disease, thalassemia trait, and sideroblastic anemia showing ferritin, TIBC, serum iron, and transferrin saturation values</image>

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## Hemolytic Anemias

### Hallmarks of Hemolysis

The hallmarks include elevated LDH, elevated indirect bilirubin, elevated reticulocyte count, and decreased haptoglobin (consumed by binding free hemoglobin). The peripheral smear may show schistocytes in microangiopathic processes or spherocytes in autoimmune or hereditary conditions. Hemoglobinuria with dark urine occurs in severe intravascular hemolysis.

### Coombs (DAT)-Positive Hemolytic Anemia (Immune-Mediated)

Warm autoimmune hemolytic anemia (AIHA) is IgG-mediated and associated with lymphoproliferative disorders, autoimmune diseases such as SLE, and medications. Treatment includes corticosteroids, rituximab, and splenectomy. Cold agglutinin disease is IgM-mediated and associated with Mycoplasma infection and lymphoproliferative disorders. Management involves avoiding cold exposure and rituximab for chronic disease; splenectomy is ineffective. Drug-induced hemolysis can be caused by penicillins, cephalosporins, and methyldopa.

### Coombs-Negative Hemolytic Anemia

Microangiopathic hemolytic anemia (MAHA) presents with schistocytes on the peripheral smear. TTP results from ADAMTS13 deficiency and produces the classic pentad of thrombocytopenia, MAHA, neurologic changes, renal dysfunction, and fever, though the full pentad is rare. Treatment is plasma exchange, caplacizumab, and steroids. HUS can be Shiga toxin-associated (typical) or complement-mediated (atypical). DIC involves widespread activation of coagulation. Other causes of MAHA include HELLP syndrome, malignant hypertension, and mechanical heart valves.

Hereditary causes include hereditary spherocytosis, G6PD deficiency (which causes episodic hemolysis with oxidative stress from infections, fava beans, or medications), and sickle cell disease. Paroxysmal nocturnal hemoglobinuria (PNH) is complement-mediated and diagnosed by flow cytometry for GPI-anchored proteins, with treatment using eculizumab or ravulizumab.

### Workup of Hemolysis

The workup includes a CBC with peripheral smear, reticulocyte count, LDH, haptoglobin, indirect bilirubin, and direct antiglobulin test (DAT/Coombs). If the DAT is negative, consider MAHA (look for schistocytes), PNH (flow cytometry), and hereditary causes (osmotic fragility, G6PD level, hemoglobin electrophoresis).

<image>Diagnostic approach to hemolytic anemia showing DAT-positive (warm AIHA, cold agglutinin, drug-induced) versus DAT-negative (MAHA, hereditary, PNH) pathways with key laboratory findings</image>

## Pancytopenia

Pancytopenia is a reduction in all three cell lines. Causes include bone marrow failure (aplastic anemia, MDS, leukemia, myelofibrosis), megaloblastic anemia from severe B12 or folate deficiency, hypersplenism, infections (HIV, TB, viral), and medications. The workup includes a peripheral smear and often a bone marrow biopsy. Urgent hematology referral is indicated if marrow failure or malignancy is suspected.

## Clinical Pearls

Ferritin is an acute-phase reactant, and in the setting of inflammation, a ferritin up to 100 (some experts say 200) does not exclude iron deficiency. A transferrin saturation below 20% supports concurrent IDA in these cases. IV iron is more effective than oral iron for most hospitalized patients with IDA, especially in CKD and inflammatory states. A mixed deficiency combining iron with B12 or folate can produce a normal MCV, so always check iron studies, B12, and folate when the cause of anemia is unclear. G6PD levels should not be checked during acute hemolysis because young reticulocytes have higher G6PD activity and may give a falsely normal result; recheck after recovery. TTP is a clinical diagnosis requiring emergent plasma exchange, and treatment should not be delayed while awaiting ADAMTS13 results if clinical suspicion is high. New-onset iron deficiency in men or postmenopausal women mandates GI evaluation with endoscopy and colonoscopy to exclude malignancy.

## References
- Camaschella C. Iron-Deficiency Anemia. *N Engl J Med*. 2015;372:1832-1843.
- Means RT. Pure Red Cell Aplasia. *Blood*. 2016;128:2504-2509.
- Green R, Datta Mitra A. Megaloblastic Anemias. *Med Clin North Am*. 2017;101:169-193.
- Hill A, et al. Paroxysmal Nocturnal Haemoglobinuria. *Nat Rev Dis Primers*. 2017;3:17028.
- Auerbach M, Adamson JW. How We Diagnose and Treat Iron Deficiency Anemia. *Am J Hematol*. 2016;91:31-38.