Complete Blood Count (CBC) — Purpose, Procedure & Normal Values | MyMedicPlus

A complete blood count (CBC) — also called a full blood count (FBC) in the United Kingdom and Australia — is one of the most widely ordered blood tests in clinical medicine. It systematically analyses the three major cellular components of blood: red blood cells (RBCs, or erythrocytes), which transport oxygen and carbon dioxide using haemoglobin; white blood cells (WBCs, or leukocytes), which defend the body against infection, inflammation, and malignancy; and platelets (thrombocytes), which initiate the clotting cascade to prevent haemorrhage. In addition to raw cell counts, the CBC measures haemoglobin concentration, haematocrit (the proportion of blood volume occupied by RBCs), and a series of derived indices — mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), and red cell distribution width (RDW). A differential white cell count identifies the proportions of neutrophils, lymphocytes, monocytes, eosinophils, and basophils. The CBC is ordered as a first-line investigation by virtually every medical specialty and provides essential diagnostic and monitoring information across haematology, oncology, infectious disease, and internal medicine.

The CBC is ordered to diagnose, classify, and monitor a wide spectrum of conditions. For RBC parameters: anaemia (low haemoglobin), polycythaemia vera (elevated RBC mass), and hereditary haemolytic disorders such as sickle cell disease and thalassaemia. For WBC parameters: bacterial or viral infection (elevated WBC with appropriate differential shift), sepsis monitoring, eosinophilia from allergy or parasitic infection, leukaemia detection (extreme or morphologically abnormal WBC counts with blast cells), and immunosuppression monitoring in patients receiving chemotherapy or immunosuppressant drugs. For platelets: thrombocytopaenia (low platelets causing bleeding risk) from immune thrombocytopaenic purpura (ITP), aplastic anaemia, or chemotherapy; thrombocytosis (elevated platelets) in reactive states or myeloproliferative neoplasms. The CBC is also part of routine pre-operative screening, periodic health checks, pregnancy care, and safety monitoring during treatment with disease-modifying anti-rheumatic drugs (DMARDs), antiepileptics, and antithyroid medications.

No fasting is required for a standard CBC, making it suitable as an urgent or random-time investigation. However, if the CBC is ordered as part of a broader metabolic panel that includes blood glucose, lipid profile, or liver function tests, follow the fasting instructions provided for the combined panel (typically 8–12 hours). Continue all regular medications unless your doctor explicitly advises otherwise — most medications do not meaningfully affect CBC values. Inform your doctor of any recent acute illness, severe physical exertion in the past 24 hours, blood transfusion within the past month, known haematological disorder, or current use of anticoagulants, corticosteroids, or chemotherapy agents, as all of these can alter CBC parameters. Wear a loose-sleeved top or clothing that can be easily rolled up at the elbow to allow venous access without difficulty.

A phlebotomist applies a tourniquet to the upper arm, cleans the antecubital fossa with an antiseptic swab, and inserts a sterile needle into a prominent vein to collect 3–5 mL of blood into a purple-top (EDTA — ethylenediaminetetraacetic acid) anticoagulant tube. EDTA prevents blood from clotting, preserving cellular morphology intact for automated analysis. The tube is gently inverted 6–8 times to mix the anticoagulant and transported to the haematology laboratory within 4 hours of collection. The sample is run through an automated haematology analyser that counts and sizes cells using electrical impedance (Coulter principle) and laser-based optical scattering — providing rapid, accurate cell counts and derived indices. Results are typically available within 1–4 hours. If the analyser flags morphological abnormalities (blasts, atypical lymphocytes, abnormal platelet clumping, or immature granulocytes), a haematologist reviews a manual peripheral blood film stained with Giemsa, Wright, or Leishman stain under a microscope for definitive characterisation.

Adult reference ranges: Haemoglobin — Men 13.5–17.5 g/dL, Women 12.0–15.5 g/dL (WHO defines anaemia as <13 g/dL in men and <12 g/dL in non-pregnant women). Red blood cell count — Men 4.5–5.9 million/μL, Women 4.1–5.1 million/μL. White blood cell count — 4,000–11,000 cells/μL total; Neutrophils 1,800–7,700/μL; Lymphocytes 1,000–4,800/μL; Monocytes 200–900/μL; Eosinophils 0–450/μL. Platelet count — 150,000–400,000/μL; below 50,000/μL carries significant bleeding risk. Haematocrit — Men 41–53%, Women 36–46%. MCV — 80–100 femtolitres (fL): low MCV (<80 fL) indicates microcytic anaemia (iron deficiency, thalassaemia); high MCV (>100 fL) indicates macrocytic anaemia (B12 or folate deficiency, alcohol, hypothyroidism). MCHC: 32–36 g/dL. RDW: 11.5–14.5%; elevated RDW indicates heterogeneous RBC population, seen in mixed nutritional deficiencies. A WBC above 30,000/μL with morphological abnormalities warrants urgent haematology review for possible leukaemia.

The CBC is an extremely safe test. Minor bruising, localised soreness, or a small haematoma at the venepuncture site are the only common adverse effects, occurring in under 5% of blood draws and typically resolving within 24–48 hours. Prolonged tourniquet application or repeated needle attempts increase bruising risk. Blood cell counts are dynamic — they fluctuate with hydration status, physical exercise, altitude, circadian rhythm, emotional stress, and even specimen handling; counts from blood collected in the morning may differ slightly from an afternoon sample. A single isolated abnormal value must always be clinically correlated — for example, mild leukocytosis with no symptoms in a healthy young adult is often physiological stress or early infection rather than leukaemia. Automated haematology analysers cannot identify all morphological abnormalities; a peripheral blood film review is needed for accurate characterisation of blast cells, hypersegmented neutrophils, schistocytes, or other significant morphological changes. The CBC reflects the blood compartment only — bone marrow pathology requires a marrow aspirate and trephine biopsy for full characterisation.