1What Is a CBC Blood Test?
A Complete Blood Count (CBC) — also called a Full Blood Count (FBC) in the UK and many Commonwealth countries — is the most frequently ordered blood test in clinical medicine worldwide. It provides a quantitative snapshot of the three major cell lines in blood: red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
A CBC is ordered routinely as part of annual health check-ups, pre-surgical workups, and pre-anaesthetic evaluations. It is also ordered diagnostically when a patient presents with symptoms such as fatigue, weakness, fever, unusual bruising or bleeding, recurrent infections, or unexplained weight loss. The results guide investigation for conditions ranging from iron-deficiency anaemia and vitamin B12 deficiency to infections, autoimmune disorders, and haematological malignancies.
What Does a CBC Measure?
| Parameter | Full Name | Male Normal | Female Normal | Unit |
|---|---|---|---|---|
| WBC | White Blood Cell Count | 4.5–11.0 | 4.5–11.0 | ×10³/µL |
| RBC | Red Blood Cell Count | 4.5–5.9 | 4.0–5.2 | ×10⁶/µL |
| Hgb | Haemoglobin | 13.5–17.5 | 12.0–15.5 | g/dL |
| Hct | Haematocrit / PCV | 41–53 | 36–46 | % |
| MCV | Mean Corpuscular Volume | 80–100 | 80–100 | fL |
| MCH | Mean Corpuscular Haemoglobin | 27–33 | 27–33 | pg |
| MCHC | Mean Corpuscular Hgb Concentration | 32–36 | 32–36 | g/dL |
| RDW | Red Cell Distribution Width | 11.5–14.5 | 11.5–14.5 | % |
| PLT | Platelet Count | 150–400 | 150–400 | ×10³/µL |
| MPV | Mean Platelet Volume | 7.5–12.5 | 7.5–12.5 | fL |
| Neutrophils | Segmented neutrophils (differential) | 50–70 | 50–70 | % |
| Lymphocytes | Lymphocyte differential | 20–40 | 20–40 | % |
| Monocytes | Monocyte differential | 2–10 | 2–10 | % |
| Eosinophils | Eosinophil differential | 1–4 | 1–4 | % |
| Basophils | Basophil differential | 0–1 | 0–1 | % |
2Understanding Each CBC Parameter
Haemoglobin (Hgb) — The Anaemia Marker
Haemoglobin is the iron-containing protein inside red blood cells that carries oxygen from the lungs to tissues. It is the primary indicator of anaemia. Low haemoglobin (anaemia) is the most common CBC abnormality worldwide, affecting over 1.6 billion people. Causes range from iron deficiency (most common globally) to vitamin B12/folate deficiency, chronic kidney disease, haemolysis, and bone marrow suppression. High haemoglobin (polycythaemia) can indicate dehydration, lung disease, living at high altitude, or polycythaemia vera.
MCV — The Anaemia Classifier
Mean Corpuscular Volume measures average red blood cell size and is the cornerstone of anaemia classification. Low MCV (microcytic anaemia, <80 fL) points to iron deficiency or thalassaemia. Normal MCV (normocytic) with anaemia suggests anaemia of chronic disease, acute blood loss, or early deficiency states. High MCV (macrocytic, >100 fL) indicates vitamin B12 or folate deficiency, hypothyroidism, liver disease, alcohol use, or certain medications (methotrexate, hydroxyurea).
WBC — The Infection and Immunity Monitor
The white blood cell count reflects immune system activity. High WBC (leukocytosis, >11,000/µL) most commonly results from bacterial infection, physiological stress, corticosteroid use, or inflammation. Extremely high counts (>50,000/µL) raise concern for leukaemia. Low WBC (leukopenia, <4,500/µL) can indicate viral infections, autoimmune disease, bone marrow suppression, or medication effects (chemotherapy, certain antibiotics).
Neutrophils vs Lymphocytes — The Differential
Neutrophilia (high neutrophil %) indicates bacterial infection, acute stress, or steroid use. Neutropenia (<1,500 absolute) significantly increases infection risk. Lymphocytosis (high lymphocytes) is typical in viral infections — especially EBV (mononucleosis), CMV, and HIV. Lymphopenia is seen in HIV, lupus, and steroid use. Eosinophilia (>4%) suggests parasitic infection, allergic conditions (asthma, urticaria), or drug reactions.
Platelets — The Bleeding and Clotting Indicator
Thrombocytopenia (low platelets, <150,000/µL) is a critical finding. Mild thrombocytopenia (100–150K) requires monitoring. Moderate (50–100K) increases surgical bleeding risk. Severe (<50K) is a bleeding risk; <20K may require platelet transfusion. Common causes include dengue fever, malaria, ITP, liver disease, and chemotherapy. Thrombocytosis (>400,000/µL) is commonly reactive (infection, iron deficiency, post-splenectomy) and rarely due to essential thrombocythaemia.
RDW — The Red Cell Size Variability
Red Cell Distribution Width measures the variability in red cell size (anisocytosis). A high RDW (>14.5%) with low MCV strongly suggests iron deficiency anaemia. High RDW with high MCV indicates mixed deficiency (iron + B12/folate) or myelodysplastic syndrome. Normal RDW with low MCV suggests thalassaemia trait. RDW is a valuable tool for distinguishing between iron deficiency and thalassaemia without further testing.
3Frequently Asked Questions
A Complete Blood Count (CBC) evaluates red blood cells, white blood cells, and platelets, serving as an indispensable screening and diagnostic test across all branches of clinical medicine.