Complete Blood Count (CBC): White Blood Cells & Red Blood Cells

What is a CBC?

Hematopoiesis — Origin of Blood Cells

Part 1: White Blood Cells (Leukocytes) in CBC

1. Total WBC Count (Leukocyte Count)

• Infection, burns, bone fractures, other bodily injuries (inflammatory response)
• Strenuous exercise, stress, pregnancy and labor
• Glucocorticoid use (a common non-infectious cause)
• Hyperleukocytosis (>100 × 10⁹ cells/L) — commonly indicates leukemia
• In biliary disease: WBC >18,000/mm³ is classified as Grade II (moderate) acute cholecystitis per 2018 Tokyo Guidelines

• Radiation or chemotherapy
• Autoimmune diseases
• Bone marrow disease (aplastic anemia)
• Specific drugs: antipsychotics, antiepileptics, immunosuppressives
• HIV/AIDS

2. WBC Differential (Leukocyte Differential)

"Absolute cell counts can provide valuable information. Low absolute neutrophil count can point to immunodeficiencies associated with neutropenia." — Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1175

Neutrophils

• First responders to bacterial infection and tissue injury; they phagocytose pathogens
• Elevated in acute bacterial infections, inflammation, stress response
• Low absolute neutrophil count (ANC) → points to neutropenia; associated with immunodeficiencies
• Hypersegmented neutrophils on smear → folate or vitamin B12 deficiency
• Large vacuoles in neutrophils → seen in Chediak-Higashi syndrome
• Neutrophils migrate out of circulation rapidly after entering from bone marrow, spending most of their life in tissues

Lymphocytes

• Include T cells, B cells, and NK cells
• At minimum, regardless of age: >1,500 small lymphocytes/mm³ expected
• Because only ~10% of circulating lymphocytes are B lymphocytes, an absolute lymphocyte deficiency primarily reflects T-cell deficiency
• Lymphocytosis with abnormal morphology (increased cytoplasm, prominent nucleoli) → infectious mononucleosis
• Hairy cell lymphocytes → seen in hairy cell leukemia
• Small lymphocytes (<10 μm) are the predominant type assessed on CBC/differential

Monocytes

• Largest circulating leukocyte; mature into macrophages and dendritic cells in tissues
• Low absolute monocyte count → seen in GATA2 deficiency
• Elevated in chronic infections, inflammatory conditions, and certain malignancies

Eosinophils

• Involved in allergic responses, parasitic infections, and tissue inflammation
• Elevated absolute eosinophil count → seen in:
  • Eosinophilic granulomatosis with polyangiitis (Churg-Strauss): a count >1,000 cells/μL helps differentiate this from asthma and atopy
  • Omenn syndrome, hyper-IgE syndrome (HIES), and other primary immunodeficiencies
• Eosinophilia is also associated with drug reactions and parasitic infestations

Basophils

• Least common circulating WBC
• Involved in allergic reactions and hypersensitivity responses (release histamine and heparin)
• Elevated in myeloproliferative disorders (especially chronic myelogenous leukemia)

3. CBC WBC Findings in Specific Diseases

Part 2: Red Blood Cells (Erythrocytes) in CBC

1. RBC Count

• Primary polycythemia: intrinsic bone marrow overproduction (e.g., polycythemia vera, primary familial and congenital polycythemia [PFCP] — genetic)
• Secondary polycythemia: increased erythropoietin (EPO) production in response to chronic hypoxia, high altitude, or EPO-secreting tumor

• Blood loss (external/internal hemorrhage) — note: in acute hemorrhage, Hgb/HCT lag behind actual RBC loss due to equilibration time
• Iron deficiency, vitamin B12 deficiency, poor nutrition
• Pregnancy, chronic disease
• Genetic disorders (e.g., sickle cell anemia)
• Autoimmune hemolytic anemia (e.g., Wiskott-Aldrich syndrome)

2. Hemoglobin (Hgb)

• Reflects the erythrocyte's ability to carry oxygen
• Along with hematocrit, it is one of the two major tests used to evaluate for anemia or polycythemia
• Hemoglobin disorders (e.g., sickle cell, thalassemias) are assessed in the context of CBC findings

3. Hematocrit (HCT) / Packed Cell Volume (PCV)

• Measures the percentage of erythrocyte volume in the blood sample
• Determined by centrifugation of a blood sample
• Approximately 45% of blood volume is cells; 55% is plasma
• A key parameter alongside Hgb for anemia/polycythemia workup

4. Erythrocyte Indices

5. Peripheral Blood Smear — RBC Morphology

6. Reticulocyte Count

• While not always part of the standard CBC, reticulocytes (immature RBCs) are often reported alongside
• Elevated reticulocyte count → active erythropoiesis (response to blood loss or hemolysis)
• Low reticulocyte count in the setting of anemia → inadequate bone marrow response (aplastic anemia, nutritional deficiency)

CBC Summary Reference Table

• Histology: A Text and Atlas with Correlated Cell and Molecular Biology, pp. 667, 728–732
• Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1175
• Rosen's Emergency Medicine: Concepts and Clinical Practice
• Fitzpatrick's Dermatology, p. 2577
• Sabiston Textbook of Surgery
• Smith and Tanagho's General Urology, 19th Edition