Medical School · Year 2 · Hematology Oncology · includes a quiz and discussion video

Lecture 7: White Blood Cell Disorders

Unit 2.9: Hematology and Oncology

Learning Objectives

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

1. Describe the evaluation of abnormal white blood cell counts
2. Explain causes and consequences of neutropenia
3. Describe causes of neutrophilia and lymphocytosis
4. Explain lymphopenia and its clinical significance
5. Describe eosinophilia and its causes
6. Explain reactive vs malignant WBC disorders

Lecture Outline

I. Normal WBC Values and Differential

The complete blood count with differential provides essential information about circulating white blood cells, which serve as the primary defenders against infection and malignancy. The total white blood cell count in healthy adults typically ranges from 4,500 to 11,000 cells per microliter, with neutrophils comprising the majority at 2,500 to 7,500 per microliter representing approximately 50 to 70 percent of circulating leukocytes. Lymphocytes constitute the second largest population at 1,000 to 4,000 per microliter accounting for 20 to 40 percent, while monocytes range from 200 to 800 per microliter at 2 to 8 percent of the total count. Eosinophils normally number 50 to 500 per microliter representing 1 to 4 percent, and basophils are the rarest circulating leukocyte at 0 to 100 per microliter comprising less than 1 percent of white blood cells.

Age-related variations in white blood cell counts and differential are clinically important and must be considered when interpreting laboratory results in pediatric patients. Neonates typically demonstrate higher total white blood cell counts with initial neutrophil predominance during the first few days of life, followed by a transition to lymphocyte predominance that characterizes infancy and childhood. This lymphocyte predominance persists through approximately age 4 to 8 years, after which the adult pattern of neutrophil predominance gradually emerges and continues throughout adulthood. Elderly patients may demonstrate slightly lower white blood cell counts compared to younger adults, though this finding requires careful clinical correlation to exclude underlying pathology.

Leukocytosis refers to an elevated white blood cell count exceeding 11,000 per microliter, while leukopenia describes a reduced count below 4,500 per microliter, and each finding requires systematic evaluation to identify the affected cell lineage. The differential count expressed in absolute numbers rather than percentages provides more clinically meaningful information, as percentage changes may be misleading when total counts are abnormal. Absolute counts are calculated by multiplying the percentage of each cell type by the total white blood cell count, allowing accurate assessment of whether specific populations are truly elevated or decreased. This distinction is particularly important when evaluating potential neutropenia, where absolute neutrophil count directly correlates with infection risk.

The approach to abnormal white blood cell counts requires systematic evaluation beginning with confirmation of the abnormality through repeat testing when appropriate. Review of the differential with attention to absolute counts identifies which specific cell population is affected, and peripheral blood smear examination provides morphologic information that may suggest underlying etiology. Clinical context including recent infections, medications, and systemic symptoms guides the diagnostic evaluation, and appropriate additional testing may include flow cytometry, bone marrow examination, or molecular studies depending on the suspected diagnosis. This methodical approach ensures accurate diagnosis while avoiding unnecessary testing in clearly reactive conditions.

<image>Panel A: Normal WBC differential showing neutrophil predominance in adults versus lymphocyte predominance in children. Panel B: Age-related variations in WBC counts from neonatal period through adulthood. Panel C: Comparison of absolute versus percentage counts demonstrating the importance of absolute count calculation. Panel D: Algorithm for systematic approach to abnormal WBC evaluation.</image>

II. Neutropenia

Neutropenia is defined by a reduction in the absolute neutrophil count below the normal range, with severity classifications that directly correlate with infection risk and guide clinical management decisions. Mild neutropenia encompasses ANC values of 1,000 to 1,500 per microliter and typically poses minimal infection risk in otherwise healthy individuals. Moderate neutropenia refers to ANC between 500 and 1,000 per microliter and carries increased but still manageable infection risk, while severe neutropenia with ANC below 500 per microliter represents a significant infectious risk requiring heightened vigilance. Agranulocytosis describes the most severe form with ANC below 200 per microliter, where infection risk is profound and patients may develop life-threatening bacterial or fungal infections rapidly.

The mechanisms underlying neutropenia can be categorized by pathophysiology, which helps direct diagnostic evaluation and treatment selection. Decreased production occurs with bone marrow failure from aplastic anemia, marrow infiltration by malignancy or fibrosis, myelosuppressive drugs including chemotherapy, and nutritional deficiencies of vitamin B12 or folate. Increased peripheral destruction results from autoimmune neutropenia where antibodies target neutrophil surface antigens, drug-induced immune destruction through hapten mechanisms, and alloimmune neutropenia in neonates exposed to maternal antibodies. Margination or sequestration in the spleen causes redistribution neutropenia seen in hypersplenism, while increased peripheral utilization during overwhelming sepsis can temporarily deplete circulating neutrophils faster than marrow production can compensate.

Drug-induced neutropenia represents one of the most common causes encountered in clinical practice and results from either dose-dependent myelosuppression or idiosyncratic immune-mediated destruction. Chemotherapeutic agents cause predictable dose-dependent neutropenia that is expected and managed with growth factor support and infection prophylaxis during nadir periods. Antibiotics including trimethoprim-sulfamethoxazole, beta-lactams, and vancomycin may cause neutropenia through various mechanisms, while antithyroid medications such as methimazole and propylthiouracil carry specific warnings for agranulocytosis risk. Clozapine used for treatment-resistant schizophrenia requires mandatory regular monitoring due to significant agranulocytosis risk, and anticonvulsants including carbamazepine and phenytoin along with various NSAIDs have been implicated in idiosyncratic neutropenia reactions.

The clinical consequences of neutropenia relate directly to the degree of neutrophil reduction and duration of neutropenic episodes. Infection risk increases progressively as ANC decreases, with the most severe risk occurring below 500 per microliter and particularly below 100 per microliter. Bacterial infections are most common, including both gram-negative organisms such as Pseudomonas and Enterobacteriaceae and gram-positive organisms including Staphylococcus and Streptococcus species. Fungal infections become significant concerns with prolonged neutropenia exceeding 7 days, particularly invasive aspergillosis and candidiasis. Clinical presentations may be atypical because neutropenic patients cannot mount normal inflammatory responses, meaning fever may be the only sign of serious infection without typical findings of pus formation, abscess, or local inflammatory changes at mucosal sites.

<image>Panel A: Severity classification of neutropenia with corresponding infection risk at each ANC level. Panel B: Mechanisms of neutropenia illustrating decreased production, increased destruction, margination, and utilization. Panel C: Common drug classes causing neutropenia with their mechanisms of action. Panel D: Clinical manifestations of neutropenic infections showing atypical presentations.</image>

III. Specific Neutropenic Conditions

Febrile neutropenia represents a medical emergency defined by the combination of absolute neutrophil count below 500 per microliter with fever of 38.3 degrees Celsius or higher, or sustained temperature of 38 degrees Celsius or above. This condition requires immediate evaluation and empiric antibiotic therapy because neutropenic patients lack the ability to contain infections and can deteriorate rapidly to septic shock within hours. The initial evaluation includes blood cultures from peripheral sites and central lines if present, chest radiograph, urinalysis, and site-specific workup based on localizing symptoms. Treatment protocols mandate prompt administration of antipseudomonal beta-lactam antibiotics such as cefepime, piperacillin-tazobactam, or meropenem, with addition of vancomycin for specific indications including suspected catheter-related infection, known colonization with methicillin-resistant organisms, or hemodynamic instability.

Congenital neutropenias comprise a heterogeneous group of inherited disorders characterized by chronic neutropenia presenting in infancy or early childhood. Severe congenital neutropenia, historically termed Kostmann syndrome, manifests with very low absolute neutrophil counts and recurrent serious bacterial infections beginning in the first months of life, most commonly caused by ELANE gene mutations affecting neutrophil elastase. Cyclic neutropenia demonstrates a characteristic 21-day oscillation pattern with ANC reaching nadir levels for 3 to 5 days during each cycle, during which patients are susceptible to infections, followed by recovery and normal counts. Shwachman-Diamond syndrome combines neutropenia with exocrine pancreatic insufficiency and skeletal abnormalities, representing a bone marrow failure syndrome with risk for myelodysplastic syndrome and leukemia transformation.

Autoimmune neutropenia occurs when antibodies directed against neutrophil surface antigens cause accelerated peripheral destruction of neutrophils. Primary autoimmune neutropenia typically affects infants and young children and usually resolves spontaneously within one to two years, with most children experiencing mild infections managed conservatively during the neutropenic period. Secondary autoimmune neutropenia occurs in the context of systemic autoimmune diseases including systemic lupus erythematosus, rheumatoid arthritis, and particularly Felty syndrome characterized by the triad of rheumatoid arthritis, splenomegaly, and neutropenia. Large granular lymphocyte leukemia represents a clonal expansion of CD8-positive T cells or natural killer cells that causes chronic neutropenia along with variable anemia and splenomegaly.

Treatment of neutropenia depends on the underlying cause, severity, and clinical context including presence or risk of infection. Granulocyte colony-stimulating factor (G-CSF) including filgrastim and pegfilgrastim stimulates neutrophil production and is indicated for chemotherapy-induced neutropenia, congenital neutropenias, and selected cases of severe chronic neutropenia. Drug-induced neutropenia requires immediate discontinuation of the offending agent, which typically results in neutrophil recovery within one to three weeks depending on the mechanism. Treatment of underlying diseases is essential for secondary causes, including immunosuppressive therapy for autoimmune conditions or treatment of infections contributing to neutropenia. Antibiotic prophylaxis may be considered for patients with prolonged severe neutropenia, and febrile neutropenia always requires immediate empiric antibiotics regardless of the underlying etiology.

<image>Panel A: Febrile neutropenia emergency protocol showing evaluation and treatment algorithm. Panel B: Features of congenital neutropenia syndromes including cyclic patterns and associated findings. Panel C: Comparison of primary and secondary autoimmune neutropenia. Panel D: Treatment approach based on etiology and severity.</image>

IV. Neutrophilia

Neutrophilia is defined as an absolute neutrophil count exceeding 7,500 per microliter and represents one of the most common abnormalities encountered on complete blood count, most frequently caused by bacterial infection. The pathophysiology involves increased marrow production in response to inflammatory signals, release of marginated neutrophils from blood vessel walls into the circulating pool, and release of marrow storage pools during acute stress responses. Infection, particularly bacterial infection, stimulates neutrophilia through cytokine-mediated effects on bone marrow including interleukin-1, interleukin-6, and granulocyte colony-stimulating factor released from macrophages and other inflammatory cells. Inflammatory conditions beyond infection also drive neutrophilia, including tissue necrosis from myocardial infarction or burns, surgical trauma, pancreatitis, and inflammatory bowel disease.

Medications represent an important and often overlooked cause of neutrophilia in clinical practice. Corticosteroids cause neutrophilia through multiple mechanisms including demargination of neutrophils from blood vessel walls, release from bone marrow storage pools, and impaired egress from blood into tissues. Lithium used for bipolar disorder stimulates neutrophil production and may be used therapeutically in some neutropenic conditions. Exogenous G-CSF administration predictably causes neutrophilia as part of its therapeutic effect, and catecholamines including epinephrine cause rapid neutrophilia through demargination, explaining the neutrophilia observed with stress responses. Malignancy, both solid tumors producing colony-stimulating factors and primary myeloproliferative neoplasms, must be considered when neutrophilia is unexplained or extreme.

A left shift describes the presence of increased immature neutrophil forms in peripheral blood, including band forms, metamyelocytes, and occasionally myelocytes, indicating active bone marrow response to increased demand. This finding suggests ongoing significant infection or inflammation requiring urgent production of neutrophils beyond the mature forms normally released. Leukemoid reaction represents an extreme form of reactive neutrophilia with white blood cell counts exceeding 50,000 per microliter that may initially raise concern for leukemia. Leukemoid reactions occur with severe infections, significant hemorrhage, or marked hemolysis, and peripheral smear examination reveals left shift with toxic granulation and Dohle bodies in neutrophils, distinguishing this reactive process from chronic myeloid leukemia where the leukocyte alkaline phosphatase score is characteristically low rather than elevated.

Leukoerythroblastic reaction describes the simultaneous presence of nucleated red blood cells and immature white blood cells in peripheral blood, a finding that suggests bone marrow infiltration or disruption. This pattern, also termed myelophthisis, occurs when normal marrow architecture is displaced by infiltrating malignancy including metastatic solid tumors, myelofibrosis, or granulomatous diseases. The peripheral smear characteristically demonstrates tear-drop red blood cells (dacrocytes) along with nucleated erythroid precursors and immature myeloid cells, creating a distinctive picture that should prompt bone marrow evaluation. Distinguishing reactive neutrophilia from myeloproliferative neoplasms requires integration of clinical context, leukocyte alkaline phosphatase scoring (elevated in reactive conditions, low in CML), cytogenetic studies for Philadelphia chromosome, and molecular testing for JAK2 or other driver mutations.

<image>Panel A: Causes of neutrophilia organized by mechanism including infection, inflammation, drugs, and malignancy. Panel B: Left shift morphology showing band forms, metamyelocytes, and toxic granulation. Panel C: Comparison of leukemoid reaction versus chronic myeloid leukemia features. Panel D: Leukoerythroblastic picture with tear-drop cells and nucleated red cells indicating marrow infiltration.</image>

V. Lymphocytosis

Lymphocytosis is defined as an elevated absolute lymphocyte count exceeding 4,000 per microliter in adults, though higher thresholds apply to children who normally have lymphocyte-predominant differentials with counts up to 7,000 per microliter considered normal. The evaluation of lymphocytosis requires determination of whether the increased lymphocytes represent a reactive polyclonal response to infection or inflammation versus a clonal malignant proliferation requiring hematologic evaluation. Reactive lymphocytosis typically occurs in younger patients, is self-limited, and resolves with treatment or resolution of the underlying infectious or inflammatory trigger. Clonal lymphocytosis should be suspected in older patients, when lymphocyte counts are extremely elevated, when lymphocytes appear morphologically abnormal, or when lymphocytosis persists without clear reactive cause.

Reactive or polyclonal lymphocytosis most commonly results from viral infections, which stimulate vigorous T-lymphocyte responses as part of normal antiviral immunity. Epstein-Barr virus causing infectious mononucleosis is the classic example, producing marked lymphocytosis with characteristic atypical lymphocytes in adolescents and young adults presenting with pharyngitis, lymphadenopathy, and splenomegaly. Cytomegalovirus causes a similar heterophile-negative mononucleosis syndrome, while acute HIV infection, viral hepatitis, and toxoplasmosis also produce reactive lymphocytosis. Bacterial infections can cause lymphocytosis, most notably pertussis infection producing extremely elevated lymphocyte counts that may exceed 50,000 per microliter in children, and tuberculosis and brucellosis causing chronic lymphocytosis in endemic regions.

Atypical lymphocytes are large lymphocytes with irregular nuclear contours, abundant basophilic cytoplasm that often appears scalloped where it contacts adjacent red blood cells, and prominent cytoplasmic granules in some cases. Despite their name suggesting abnormality, atypical lymphocytes represent activated CD8-positive cytotoxic T cells mounting an immune response and are not malignant cells. Infectious mononucleosis from Epstein-Barr virus classically produces abundant atypical lymphocytes that may comprise 10 to 30 percent or more of peripheral blood lymphocytes. Other causes of atypical lymphocytosis include cytomegalovirus infection, acute HIV, viral hepatitis, and drug hypersensitivity reactions, making the finding nonspecific but suggestive of immune activation.

Clonal or malignant lymphocytosis requires consideration when lymphocytes appear morphologically abnormal on smear, when lymphocytosis is extreme without clear reactive cause, or when it occurs in elderly patients. Chronic lymphocytic leukemia is the most common cause of clonal lymphocytosis and presents with mature-appearing but fragile lymphocytes that produce characteristic smudge cells on the blood smear in elderly patients. Other lymphoproliferative disorders that may present with blood involvement include various lymphomas, adult T-cell leukemia associated with HTLV-1, and large granular lymphocyte leukemia. Flow cytometry is essential for distinguishing reactive from clonal lymphocytosis by demonstrating immunophenotype consistent with specific disorders and detecting light chain restriction or aberrant marker expression indicative of clonality.

<image>Panel A: Reactive versus clonal lymphocytosis with distinguishing clinical and laboratory features. Panel B: Atypical lymphocyte morphology showing characteristic large cells with basophilic cytoplasm and irregular nuclei. Panel C: Classic features of infectious mononucleosis including pharyngitis, lymphadenopathy, and atypical lymphocytes. Panel D: Chronic lymphocytic leukemia demonstrating smudge cells and mature lymphocyte morphology.</image>

VI. Lymphopenia

Lymphopenia is defined as an absolute lymphocyte count below 1,000 per microliter in adults and indicates depletion of the adaptive immune system's primary cellular component. This finding is clinically significant because lymphocytes mediate both cellular immunity essential for defense against viruses, fungi, and mycobacteria, and humoral immunity through B-cell antibody production. The causes of lymphopenia are diverse and include viral infections that directly target lymphocytes, autoimmune disorders, medications, malignancy, and congenital immunodeficiency syndromes. Evaluation requires consideration of the clinical context, HIV testing when appropriate, and assessment for underlying systemic disease.

Infectious causes of lymphopenia include both acute viral infections and chronic infectious diseases that deplete lymphocyte populations through various mechanisms. HIV infection directly infects and destroys CD4-positive T lymphocytes, making the CD4 count a critical measure of immune status and risk for opportunistic infections. COVID-19 characteristically causes lymphopenia during acute infection, which correlates with disease severity and outcomes. Measles virus infects lymphocytes and causes transient but significant immune suppression, while tuberculosis and other mycobacterial infections produce chronic lymphopenia through complex immunologic mechanisms including compartmentalization of lymphocytes to sites of infection.

Autoimmune diseases frequently cause lymphopenia through immune-mediated lymphocyte destruction and altered lymphocyte trafficking. Systemic lupus erythematosus characteristically produces lymphopenia, which is included as a diagnostic criterion and results from anti-lymphocyte antibodies and other immunologic mechanisms. Rheumatoid arthritis and other systemic autoimmune conditions also demonstrate variable lymphopenia. Medications represent important causes of lymphopenia, including corticosteroids which cause lymphocyte apoptosis and redistribution, chemotherapeutic agents that are cytotoxic to proliferating lymphocytes, and specific biologic agents such as rituximab which depletes B lymphocytes through anti-CD20 targeting.

The clinical significance of lymphopenia relates to the degree of lymphocyte depletion and the specific populations affected. In HIV infection, the CD4 count provides prognostic information and guides prophylaxis decisions, with CD4 above 500 indicating mild immunosuppression, 200 to 500 representing moderate risk for certain opportunistic infections, below 200 defining AIDS with risk for Pneumocystis pneumonia and toxoplasmosis, and below 50 indicating severe immunocompromise with risk for disseminated Mycobacterium avium complex and CMV retinitis. Congenital lymphopenias include severe combined immunodeficiency presenting in infancy with life-threatening infections requiring urgent bone marrow transplantation. The opportunistic infection risk associated with lymphopenia emphasizes the critical role of lymphocytes in defense against pathogens that are controlled by cell-mediated immunity.

<image>Panel A: Causes of lymphopenia organized by mechanism including infection, autoimmune, drugs, and congenital. Panel B: HIV-associated CD4 decline with corresponding opportunistic infection risks at each threshold. Panel C: Medication-induced lymphopenia showing mechanisms of steroids, chemotherapy, and biologics. Panel D: Clinical manifestations of lymphopenia-associated immunodeficiency.</image>

VII. Eosinophilia

Eosinophilia is defined by an elevated absolute eosinophil count, with mild eosinophilia ranging from 500 to 1,500 per microliter, moderate eosinophilia from 1,500 to 5,000 per microliter, and severe eosinophilia exceeding 5,000 per microliter indicating significant elevation requiring thorough evaluation. Eosinophils are granulocytes that normally participate in defense against parasitic infections and modulate allergic and inflammatory responses, but their tissue accumulation can cause significant organ damage through release of cytotoxic granule contents. The differential diagnosis of eosinophilia is broad but can be organized using the mnemonic NAACP representing Neoplasm, Allergic/Atopic, Adrenal insufficiency, Connective tissue disease, and Parasites. Each of these categories encompasses multiple specific diagnoses that require distinct evaluation and management approaches.

Allergic and atopic conditions represent the most common cause of eosinophilia in developed countries where parasitic infections are uncommon. Allergic rhinitis, asthma, and atopic dermatitis (eczema) produce mild to moderate eosinophilia reflecting the role of eosinophils in allergic inflammation mediated by interleukin-5 and other type 2 cytokines. Drug hypersensitivity reactions may cause eosinophilia, with the most severe form being Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS syndrome), a potentially life-threatening condition with rash, fever, lymphadenopathy, and multi-organ involvement occurring weeks after drug exposure. Eosinophilic gastrointestinal disorders including eosinophilic esophagitis represent a growing category of allergic disease with tissue eosinophil infiltration causing dysphagia and other symptoms.

Parasitic infections, particularly tissue-invasive helminths, stimulate potent eosinophil responses as part of antiparasitic immunity. Intestinal parasites that remain within the gut lumen may not cause significant eosinophilia, but tissue-migrating larvae from Strongyloides, Ascaris, hookworm, and other helminths provoke marked eosinophilic responses. Visceral larva migrans from Toxocara and trichinosis from Trichinella demonstrate how tissue parasitic infection drives eosinophilia. Evaluation for parasitic causes is essential in patients with travel history to endemic regions, immigrants from tropical areas, or those with other risk factors, and requires specific serologic testing as stool examination alone may be insufficient.

Hypereosinophilic syndrome (HES) represents a category of disorders defined by persistent eosinophilia exceeding 1,500 per microliter for more than six months accompanied by eosinophil-mediated organ damage. The organs most commonly affected include the heart with eosinophilic endomyocardial fibrosis leading to restrictive cardiomyopathy, lungs with pulmonary infiltrates and respiratory symptoms, skin with various eosinophilic dermatoses, and nervous system with peripheral or central neuropathy. HES is classified into subtypes including myeloproliferative variants harboring the FIP1L1-PDGFRA fusion gene which responds dramatically to imatinib, lymphocytic variants driven by clonal T-cells producing eosinophil-stimulating cytokines, and idiopathic HES when no underlying cause is identified. Treatment of hypereosinophilic syndrome typically begins with corticosteroids, though targeted therapy with imatinib for FIP1L1-PDGFRA positive cases and newer biologic agents targeting interleukin-5 have expanded therapeutic options.

<image>Panel A: NAACP mnemonic for eosinophilia causes with examples in each category. Panel B: Tissue-invasive parasites causing eosinophilia with global distribution maps. Panel C: Hypereosinophilic syndrome organ involvement showing cardiac, pulmonary, and skin manifestations. Panel D: FIP1L1-PDGFRA fusion and targeted therapy with imatinib.</image>

VIII. Monocytosis and Basophilia

Monocytosis is defined as an absolute monocyte count exceeding 800 per microliter and reflects activation of the mononuclear phagocyte system in response to chronic infection, inflammation, or malignancy. Monocytes are circulating precursors to tissue macrophages and dendritic cells, playing essential roles in antigen presentation, phagocytosis of pathogens and debris, and orchestration of inflammatory responses. The persistence of monocytosis beyond acute illness resolution or elevation without clear inflammatory cause should prompt evaluation for underlying malignancy, particularly myelodysplastic/myeloproliferative overlap syndromes. Chronic monocytosis in the absence of obvious reactive cause warrants careful clinical evaluation and often hematology consultation.

Chronic infections represent a major cause of reactive monocytosis, particularly infections requiring sustained macrophage-mediated immune responses for containment. Tuberculosis, both pulmonary and extrapulmonary, characteristically causes monocytosis along with its role in granuloma formation. Subacute bacterial endocarditis produces monocytosis as part of the chronic inflammatory response to persistent intravascular infection. Brucellosis and other chronic bacterial infections requiring cell-mediated immunity for clearance similarly stimulate monocyte production. Autoimmune and inflammatory conditions including inflammatory bowel disease, systemic lupus erythematosus, and rheumatoid arthritis frequently demonstrate monocytosis reflecting ongoing innate immune activation.

Chronic myelomonocytic leukemia (CMML) represents the prototypical malignant cause of monocytosis and must be considered in elderly patients with persistent unexplained monocyte elevation exceeding 1,000 per microliter. CMML is classified as an MDS/MPN overlap syndrome featuring both dysplastic changes and proliferative features, with monocytosis as the defining hematologic abnormality along with variable cytopenias and dysplasia. The diagnosis requires exclusion of reactive causes, bone marrow examination demonstrating dysplastic changes in one or more lineages with less than 20 percent blasts, and absence of BCR-ABL and other specific genetic rearrangements. CMML carries risk for transformation to acute myeloid leukemia and is treated with hypomethylating agents such as azacitidine, with allogeneic stem cell transplantation considered for eligible patients.

Basophilia, defined as an absolute basophil count exceeding 100 per microliter, is uncommon and often overlooked but carries important diagnostic implications. The most significant association is with chronic myeloid leukemia, where basophilia is a characteristic finding that helps distinguish this myeloproliferative neoplasm from reactive leukocytosis. Other myeloproliferative neoplasms including polycythemia vera and primary myelofibrosis may demonstrate basophilia. Allergic conditions including chronic urticaria may cause mild basophilia, and hypothyroidism has been associated with basophil elevation in some cases. The clinical significance of these minor cell populations lies in their diagnostic utility, as monocytosis points toward chronic infection or CMML while basophilia suggests myeloproliferative disease, particularly CML.

<image>Panel A: Causes of monocytosis including chronic infections, autoimmune diseases, and malignancy. Panel B: Chronic myelomonocytic leukemia diagnostic criteria and bone marrow findings. Panel C: Basophilia in chronic myeloid leukemia showing characteristic peripheral smear. Panel D: Algorithm for evaluating persistent monocytosis or basophilia.</image>

IX. Peripheral Smear Findings

The peripheral blood smear provides essential morphologic information that complements automated cell counts and may reveal abnormalities that suggest specific diagnoses. Neutrophil morphologic changes observed during infection and inflammation include toxic granulation appearing as coarse, dark azurophilic granules reflecting increased lysosomal enzyme content, and Dohle bodies representing blue cytoplasmic inclusions of rough endoplasmic reticulum. These toxic changes indicate bone marrow response to significant infectious or inflammatory stress and support reactive rather than malignant etiology for neutrophilia. Hypersegmented neutrophils with nuclei containing six or more lobes are characteristic of megaloblastic anemia from vitamin B12 or folate deficiency, representing one of the earliest morphologic clues to this treatable condition.

Abnormal neutrophil morphology may indicate myelodysplastic syndrome or acute leukemia rather than reactive conditions. Pseudo-Pelger-Huet anomaly describes hypolobated neutrophils with bilobed nuclei resembling eyeglasses, representing acquired dysplasia in MDS that mimics the benign congenital Pelger-Huet anomaly. Hypogranular neutrophils with pale cytoplasm lacking normal granulation also suggest myelodysplasia. Auer rods are crystallized azurophilic granules appearing as pink or red rod-shaped inclusions in the cytoplasm of myeloblasts and are pathognomonic for acute myeloid leukemia, confirming myeloid lineage when present. Multiple Auer rods bundled together form faggot cells, which are characteristic of acute promyelocytic leukemia and should prompt immediate recognition and treatment initiation.

Lymphocyte morphology on peripheral smear helps distinguish reactive from clonal lymphoproliferative disorders. Atypical lymphocytes with large size, abundant basophilic cytoplasm, and irregular nuclear contours represent reactive CD8-positive T cells and are classic for infectious mononucleosis and other viral infections. Smudge cells appear as disrupted lymphocytes with bare nuclei and represent the fragile cells of chronic lymphocytic leukemia that rupture during smear preparation, serving as a useful diagnostic clue to this common leukemia. Hairy cells demonstrate fine cytoplasmic projections around the cell periphery and are diagnostic of hairy cell leukemia, while Sezary cells with convoluted cerebriform nuclei suggest cutaneous T-cell lymphoma, and flower cells with multilobed nuclei indicate adult T-cell leukemia associated with HTLV-1.

Red blood cell inclusions and morphologic abnormalities visible on peripheral smear provide additional diagnostic information beyond white blood cell analysis. Howell-Jolly bodies are nuclear remnants appearing as dark inclusions in red cells and indicate splenic hypofunction or asplenia, as the spleen normally removes these inclusions. Heinz bodies are denatured hemoglobin inclusions seen with supravital staining in glucose-6-phosphate dehydrogenase deficiency and unstable hemoglobin variants. The presence of nucleated red blood cells in peripheral blood outside the neonatal period, combined with immature white cells and tear-drop red cells, creates the leukoerythroblastic picture suggesting bone marrow infiltration. Alder-Reilly inclusions are coarse azurophilic granules in white cells seen in mucopolysaccharidoses, demonstrating how smear examination can reveal underlying metabolic disorders.

<image>Panel A: Toxic granulation and Dohle bodies in neutrophils indicating reactive changes. Panel B: Auer rods and faggot cells diagnostic of acute myeloid leukemia and APL. Panel C: Lymphocyte abnormalities including atypical lymphocytes, smudge cells, and hairy cells. Panel D: Red cell inclusions including Howell-Jolly bodies and the leukoerythroblastic picture.</image>

X. When to Suspect Malignancy

Red flag findings on complete blood count should prompt consideration of underlying hematologic malignancy and warrant further evaluation including peripheral smear review and hematology consultation. Very high white blood cell counts exceeding 50,000 per microliter without clear reactive cause such as severe infection raise concern for leukemia, particularly when accompanied by other cytopenias or abnormal cells on differential. The presence of blasts in peripheral blood is always concerning for acute leukemia and requires urgent evaluation regardless of the total white cell count. Concurrent cytopenias affecting multiple cell lines suggest bone marrow infiltration or failure, while unexplained lymphadenopathy or splenomegaly with abnormal blood counts points toward lymphoproliferative disease.

Distinguishing reactive from malignant white blood cell elevations requires integration of clinical context, laboratory findings, and morphologic features. Reactive conditions typically occur in the setting of obvious infection, inflammation, or medication use, demonstrate orderly maturation with normal progression through developmental stages, and show elevated leukocyte alkaline phosphatase scores reflecting normal neutrophil function. Malignant proliferations occur without clear reactive trigger, may demonstrate maturation arrest with accumulation of immature forms, and in the case of chronic myeloid leukemia show characteristically low leukocyte alkaline phosphatase scores. Flow cytometry can detect clonality through light chain restriction in B-cell disorders or aberrant marker expression, and cytogenetic or molecular studies identify specific genetic abnormalities that define many hematologic malignancies.

The initial workup for suspected hematologic malignancy begins with careful review of the complete blood count with differential, followed by peripheral blood smear examination by an experienced hematologist or pathologist. Flow cytometry on peripheral blood can immunophenotype abnormal populations and detect clonality, often providing presumptive diagnosis without immediate bone marrow examination. Bone marrow aspiration and biopsy remain the definitive evaluation, providing material for morphology, flow cytometry, cytogenetics, and molecular testing needed for complete diagnosis and classification. Specific cytogenetic and molecular abnormalities, such as BCR-ABL in CML or PML-RARA in APL, have profound implications for treatment selection and prognosis.

Referral criteria should guide timely involvement of hematology specialists to ensure prompt diagnosis and treatment of hematologic malignancies. Presence of circulating blasts always warrants urgent hematology consultation and evaluation, as acute leukemia may deteriorate rapidly without treatment. Unexplained cytopenias, particularly when affecting multiple cell lines or associated with macrocytosis, require bone marrow evaluation to exclude myelodysplastic syndrome or other marrow disorders. Persistent monoclonal lymphocytosis may represent early chronic lymphocytic leukemia requiring monitoring and potential treatment. Any unexplained abnormality that persists after exclusion of common reactive causes deserves investigation to avoid delayed diagnosis of treatable malignancy.

<image>Panel A: Red flags on CBC suggesting hematologic malignancy requiring urgent evaluation. Panel B: Comparison table of reactive versus malignant features in leukocytosis. Panel C: Diagnostic algorithm for suspected malignancy from initial workup through bone marrow examination. Panel D: Indications for urgent hematology referral.</image>

Summary

• Neutropenia severity directly correlates with infection risk, with absolute neutrophil count below 500 per microliter representing severe neutropenia and below 200 per microliter defining agranulocytosis
• Febrile neutropenia is a medical emergency requiring immediate broad-spectrum antibiotic administration without waiting for culture results
• Drug-induced neutropenia is among the most common causes and requires discontinuation of the offending agent
• Neutrophilia is usually reactive to infection or inflammation, but very high counts should raise concern for chronic myeloid leukemia
• Left shift indicates active marrow response with increased bands and immature forms in peripheral blood
• Lymphocytosis may be reactive with atypical lymphocytes in viral infections or clonal in conditions like chronic lymphocytic leukemia
• Atypical lymphocytes are large cells with basophilic cytoplasm representing activated T cells, classically seen in infectious mononucleosis
• Lymphopenia occurs with HIV, steroids, and autoimmune diseases, with CD4 count determining opportunistic infection risk in HIV
• Eosinophilia causes are summarized by NAACP: Neoplasm, Allergic, Adrenal insufficiency, Connective tissue disease, Parasites
• Monocytosis suggests chronic infection or CMML, while basophilia is characteristic of chronic myeloid leukemia

Key Terms

This content is subject to the MIT License. © 2024–2026 Hibbert School of Medicine.

Read this lecture as Markdown