The complete blood count (CBC) is the most commonly ordered laboratory test in clinical medicine. It quantifies the three major cellular components of blood — erythrocytes, leukocytes, and thrombocytes — and provides critical information for diagnosing, monitoring, and screening a wide range of conditions.
Sample Collection and Handling
Blood is collected by venipuncture into tubes containing EDTA (lavender-top tube), which chelates calcium to prevent coagulation. The sample must be mixed gently by inversion to ensure uniform anticoagulation and prevent clot formation. CBC specimens are stable for up to 24 hours at room temperature, though delays beyond 4 hours may affect the differential and platelet count. Capillary samples from fingerstick or heelstick are acceptable for point-of-care devices but may introduce variability due to tissue fluid contamination.
Automated Hematology Analyzers
Modern hematology analyzers use impedance, optical scatter, and fluorescence methods to count and characterize blood cells. In impedance-based counting, cells suspended in diluent pass through an aperture under vacuum; each cell displaces its volume of conductive fluid, creating a voltage pulse proportional to cell size. Optical flow cytometry focuses cells in a single-file stream past a laser beam, measuring forward scatter (cell size), side scatter (internal complexity), and fluorescence (nucleic acid content). Fluorescence-based reagents stain nucleic acids and cellular proteins, enabling differentiation of immature cells, nucleated red blood cells, and reticulocytes.
Parameters Measured
The CBC includes a quantitative panel: white blood cell count (WBC), red blood cell count (RBC), hemoglobin (Hb), hematocrit (Hct), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), red cell distribution width (RDW), platelet count, mean platelet volume (MPV), and the white blood cell differential (absolute and relative percentages of neutrophils, lymphocytes, monocytes, eosinophils, and basophils). The automated differential also reports flags for abnormal or immature cells, which prompts a manual peripheral blood smear review.
Quality Control and Calibration
Analyzers require daily quality control using commercial controls at three levels (low, normal, high). Calibration is performed at installation and after major maintenance using calibrator materials with certified values. Results are verified against delta checks (comparison with the patient’s previous results) and instrument flags. The laboratory must participate in external proficiency testing programs, such as those from CAP or UK NEQAS, to ensure inter-laboratory accuracy.
Clinical Applications
The CBC is used for anemia screening and classification (using RBC indices), infection and inflammation assessment (WBC count and differential), bleeding and clotting risk evaluation (platelet count), bone marrow function assessment (reticulocyte count), and monitoring of chemotherapy, radiation therapy, and treatment with marrow-suppressive drugs. Abnormal results drive further investigation including peripheral blood smear review, bone marrow aspiration, flow cytometry, and molecular testing.
Reference Ranges
Reference ranges vary by age, sex, ethnicity, altitude, and laboratory. Adult reference intervals generally are: WBC 4.5–11.0 × 10⁹/L, RBC 4.2–5.9 × 10¹²/L (male) and 3.8–5.2 × 10¹²/L (female), hemoglobin 13.5–17.5 g/dL (male) and 12.0–16.0 g/dL (female), hematocrit 39–49% (male) and 35–45% (female), MCV 80–100 fL, and platelet count 150–450 × 10⁹/L. Each laboratory should establish or validate its own reference intervals for the local population.
