Complete Blood Count (CBC)

A CBC is one of the most commonly ordered laboratory panels. It quantifies and characterizes the cellular and formed elements of blood - erythrocytes, leukocytes, and thrombocytes - typically analyzed by automated flow cytometry-based cell counters that examine ~10,000 cells per category for high accuracy.

Components of the CBC

1. White Blood Cell (WBC) / Leukocyte Count

Finding	Term	Common Causes
Elevated WBC	Leukocytosis	Infection, inflammation, burns, fractures, stress, pregnancy, labor
WBC >100 x 10⁹/L	Hyperleukocytosis	Leukemia
Decreased WBC	Leukopenia	Chemotherapy, radiation, autoimmune disease, aplastic anemia, antipsychotics, antiepileptics, immunosuppressants, HIV/AIDS

2. WBC Differential

The differential reports the percentage of each leukocyte type:

Neutrophils (including band cells - immature neutrophils)
Eosinophils
Basophils
Lymphocytes
Monocytes

Absolute counts are important - for example, absolute lymphocyte count primarily reflects T-cell status since ~90% of circulating lymphocytes are T cells. The minimum expected count is >1,500 small lymphocytes/mm³.

Specific differentials to know:

Low ANC (absolute neutrophil count): suggests neutropenia/immunodeficiency
Elevated eosinophils: seen in Omenn syndrome, hyper-IgE syndrome
Low monocytes: associated with GATA2 deficiency
Large vacuoles in neutrophils: Chediak-Higashi syndrome

3. Red Blood Cell (RBC) / Erythrocyte Count

Finding	Term	Common Causes
Elevated RBC	Polycythemia	Polycythemia vera (primary), chronic hypoxia, high altitude, EPO-secreting tumor (secondary)
Decreased RBC	Anemia	Bleeding, iron/B12 deficiency, poor nutrition, pregnancy, chronic disease, sickle cell anemia

4. Hemoglobin (Hgb)

Reflects the oxygen-carrying capacity of blood.

Population	Normal Range
Males	13.5 - 17.5 g/dL (140 - 180 g/L)
Females	12 - 15.5 g/dL (120 - 150 g/L)

Hgb and hematocrit (HCT) are the two key tests for evaluating anemia vs. polycythemia.

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

The percentage of total blood volume occupied by red blood cells.

6. RBC Indices (automatically calculated)

Index	What it Measures	Clinical Use
MCV (Mean Corpuscular Volume)	Size of RBCs	Micro vs. normocytic vs. macrocytic anemia
MCH (Mean Corpuscular Hemoglobin)	Amount of Hgb per average RBC	Hypochromic vs. normochromic
MCHC (Mean Corpuscular Hgb Concentration)	% Hgb concentration in average RBC	Severity of hypochromia
RDW (Red Cell Distribution Width)	Variation in RBC size/shape (anisocytosis)	Differentiating types of anemia

7. Platelet (Thrombocyte) Count

Finding	Term	Common Causes
Elevated platelets	Thrombocythemia	Bone marrow proliferative disorders, inflammation, post-splenectomy, hyposplenism
Decreased platelets	Thrombocytopenia	Leukemia, B12 deficiency, infections, hereditary syndromes, autoimmune disease, DIC, certain medications

MPV (Mean Platelet Volume): measures average platelet size; large platelets suggest increased platelet turnover (e.g., ITP).

Special finding: small platelets are seen in Wiskott-Aldrich syndrome.

How the CBC is Performed

Blood is diluted in suspension fluid and passed as a thin stream through narrow tubing. A light detector and electrical impedance sensor identify cell types by size and resistance. Modern analyzers use computer-assisted image processing. Manual microscopy is still used in select cases (e.g., to identify blast cells, atypical lymphocytes, or Howell-Jolly bodies).

Clinically Important Peripheral Smear Findings (complement to CBC)

Finding	Associated Condition
Large vacuoles in neutrophils	Chediak-Higashi syndrome
Small platelets	Wiskott-Aldrich syndrome
Howell-Jolly bodies	Functional asplenia
Thrombocytopenia + anemia	Autoimmune lymphoproliferative syndrome, CVID, 22q11.2 deletion
Autoimmune hemolytic anemia	Wiskott-Aldrich syndrome

Sources: Histology: A Text and Atlas with Correlated Cell and Molecular Biology, pp. 728-730 | Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 1175

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