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Complete Blood Count (CBC) - Broad Chapter Description

What Is a CBC?

A Complete Blood Count (CBC) is one of the most commonly ordered laboratory test panels in clinical medicine. It provides quantitative data on the cellular components of blood - erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). Modern CBCs are performed on automated blood cell counters using flow cytometry, where a thin stream of diluted blood cells passes through a narrow tube and is analyzed by light detectors and electrical impedance sensors that identify cell types by size and electrical resistance. Roughly 10,000 cells per category are counted, making the results highly accurate.

Core Components of a CBC

1. White Blood Cell (Leukocyte) Count

Leukocytosis (elevated WBC): signals inflammation, infection, burns, fractures, strenuous exercise, pregnancy, or - at very high counts (>100 x 10⁹/L, hyperleukocytosis) - leukemia.
Leukopenia (low WBC): associated with radiation/chemotherapy, autoimmune diseases, aplastic anemia, certain drugs (antipsychotics, antiepileptics, immunosuppressants), HIV/AIDS.

2. WBC Differential

Reports the percentage and absolute count of each leukocyte type:

Neutrophils (including band cells/immature forms)
Lymphocytes
Monocytes
Eosinophils
Basophils

Each subset gives distinct diagnostic information:

Low absolute neutrophil count - points to neutropenia-associated immunodeficiencies.
Low absolute lymphocyte count (< 1,500/mm³) - primarily reflects T-cell deficiency (since ~90% of circulating lymphocytes are T cells).
Elevated absolute eosinophil count - seen in Omenn syndrome, hyper-IgE syndrome, and other primary immunodeficiencies.
Low absolute monocyte count - can indicate GATA2 deficiency.

3. Red Blood Cell (Erythrocyte) Count

Polycythemia (elevated RBC): primary (e.g., polycythemia vera, familial/congenital polycythemia) or secondary (chronic hypoxia, high altitude, erythropoietin-secreting tumors).
Anemia (low RBC): caused by blood loss, iron or B12 deficiency, poor nutrition, pregnancy, chronic disease, or genetic disorders (e.g., sickle cell anemia).

4. Hematocrit (HCT / Packed Cell Volume)

Measures the percentage of blood volume occupied by red blood cells. Together with hemoglobin, it is one of the two key tests for evaluating anemia or polycythemia.

5. Hemoglobin (Hgb)

Reflects the oxygen-carrying capacity of blood.

Normal values: 13.5-17.5 g/dL in men, 12-15.5 g/dL in women.

6. Red Cell Indices

MCV (Mean Corpuscular Volume): classifies anemia as microcytic, normocytic, or macrocytic.
MCH (Mean Corpuscular Hemoglobin): average hemoglobin per red cell.
MCHC (Mean Corpuscular Hemoglobin Concentration): hemoglobin concentration relative to cell volume.
RDW (Red Cell Distribution Width): measures variation in red cell size; elevated in iron-deficiency and mixed anemias.

7. Platelet Count & Indices

Thrombocytopenia (low platelets): seen in various conditions including autoimmune lymphoproliferative syndrome, common variable immunodeficiency, and chromosome 22q11.2 deletion syndrome. Small platelets specifically suggest Wiskott-Aldrich syndrome.
Thrombocytosis (high platelets): reactive or primary (essential thrombocythemia).

The Peripheral Blood Smear - A Key Companion

Beyond the automated count, a manual peripheral smear provides morphological detail that machines may miss:

Large vacuoles in neutrophils - Chediak-Higashi syndrome
Small platelets - Wiskott-Aldrich syndrome
Howell-Jolly bodies (nuclear remnants in RBCs) - functional asplenia
Band cells / left shift - active bacterial infection

Clinical Significance Summary

Parameter	High	Low
WBC	Infection, leukemia, inflammation	Immunodeficiency, bone marrow failure
Neutrophils	Bacterial infection, stress	Chemotherapy, autoimmune, drug reaction
Lymphocytes	Viral infection, lymphoma	T-cell deficiency, HIV
Eosinophils	Allergy, parasites, some immunodeficiencies	-
RBC/Hgb/HCT	Polycythemia	Anemia
Platelets	Reactive, essential thrombocythemia	Bleeding disorders, ITP, bone marrow failure

How It's Performed

Blood sample collected in an EDTA (purple-top) tube.
Loaded into an automated hematology analyzer using flow cytometry.
Results generated in minutes; flagged abnormal values trigger manual smear review.

Sources: Histology: A Text and Atlas with Correlated Cell and Molecular Biology, pp. 728-729; Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1175

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