Complete Blood Count (CBC)
Also known as: Full blood count (FBC), Haemogram
A CBC measures all three types of blood cells — red cells, white cells, and platelets — along with haemoglobin and haematocrit. It is one of the most ordered tests in medicine because it screens for dozens of conditions in a single draw.
Venous blood (EDTA tube)
2–4 hours
Not required
Blood Count
Reference Ranges
| Measurement | Reference Range | Unit |
|---|---|---|
| Haemoglobin (men) | 13.5 – 17.5 | g/dL |
| Haemoglobin (women) | 12 – 15.5 | g/dL |
| RBC (men) | 4.5 – 5.9 | ×10¹²/L |
| RBC (women) | 4 – 5.2 | ×10¹²/L |
| WBC | 4 – 11 | ×10⁹/L |
| Platelets | 150 – 400 | ×10⁹/L |
| Haematocrit (men) | 41 – 53 | % |
| Haematocrit (women) | 36 – 46 | % |
| MCV | 80 – 100 | fL |
Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.
What This Test Measures
The CBC quantifies every major cell type in circulating blood. Red blood cells (RBCs) carry oxygen via haemoglobin; the haematocrit tells you what percentage of your blood volume is occupied by RBCs. White blood cells (WBCs) are the immune system's first responders. Platelets plug vessel injuries and initiate clotting. The mean corpuscular volume (MCV) describes average RBC size — small cells suggest iron deficiency; large cells suggest B12 or folate deficiency.
When Is This Test Ordered?
- Routine health screening
- Suspected anaemia (fatigue, pallor, shortness of breath)
- Monitoring chemotherapy or radiation effects on bone marrow
- Suspected infection or immune disorder
- Unexplained bruising or prolonged bleeding
- Before surgery to ensure safe anaesthesia
When Results Are High
A high WBC (leukocytosis) usually signals infection, inflammation, or — rarely — leukaemia. High RBCs/haematocrit (polycythaemia) can thicken the blood and raise clot risk. High platelets (thrombocytosis) may be reactive (iron deficiency, infection) or rarely a bone-marrow disorder.
When Results Are Low
Low haemoglobin or RBC count is anaemia — the most common blood disorder globally. Low WBCs (leukopenia) weaken infection defence and can result from viral illness, autoimmune disease, or bone-marrow suppression. Low platelets (thrombocytopenia) raise bleeding risk and warrant urgent evaluation below 50 ×10⁹/L.
Understanding the differential WBC count
Most CBC reports include a five-part differential that breaks the total WBC count into subtypes. Neutrophils (normally 50–70%) are the primary bacteria-fighters; a high neutrophil count almost always means bacterial infection or stress. Lymphocytes (20–40%) drive viral immunity; a high lymphocyte count is classic for viral infections like EBV. Monocytes, eosinophils, and basophils make up the remainder.
An automated differential flags suspicious patterns for a trained scientist to review under the microscope. If your report says 'manual differential requested', it means the machine detected an abnormal cell type that needs a human eye.
MCV: the size of your red cells matters
| MCV | Cell Size | Common Causes |
|---|---|---|
| < 80 fL | Microcytic (small) | Iron deficiency, thalassaemia, chronic disease |
| 80–100 fL | Normocytic (normal) | Acute blood loss, early mixed deficiency |
| > 100 fL | Macrocytic (large) | Vitamin B12 or folate deficiency, alcohol use, hypothyroidism |
When to seek care after a CBC
- Haemoglobin below 8 g/dL — symptomatic anaemia requiring prompt evaluation
- WBC above 30 ×10⁹/L or below 1.0 ×10⁹/L — urgent haematology referral
- Platelets below 50 ×10⁹/L — significant bleeding risk
- Any 'blast cells' flagged on the report — haematology same day
Factors That Can Affect Your Result
- Altitude — people living at high altitude have higher RBC counts
- Pregnancy — plasma volume expands, diluting haemoglobin by ~10%
- Vigorous exercise can briefly raise WBCs
- Smoking raises WBC and haemoglobin
- Sample age — EDTA samples should be analysed within 4–6 hours
Tracking This Test Over Time
Track your haemoglobin trend over time in Looms to spot a slow decline months before it becomes symptomatic anaemia. A 1–2 g/dL drop across three draws is clinically significant even if each individual result stays 'in range'.
Track your CBC in LoomsRelated Tests
Iron Studies
Iron studies go beyond a simple iron level to measure the body's full iron storage and transport system — serum iron, ferritin, transferrin saturation, and TIBC — giving a complete picture of iron status.
B12
Vitamin B12 is essential for nerve function, DNA synthesis, and red blood cell production. Deficiency develops slowly — often over years — and can cause irreversible neurological damage before blood cell changes appear.
Folate
Folate is essential for DNA synthesis and cell division — it is critically important in early pregnancy for preventing neural tube defects, and together with B12 it prevents megaloblastic anaemia.
ESR
ESR measures how quickly red blood cells settle in a tube — inflammation makes them clump and fall faster. It is a non-specific but clinically useful marker for detecting and monitoring inflammatory conditions.
Medical disclaimer
This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted in the context of your symptoms, history, and other tests. Always discuss your results with your healthcare provider. Content reviewed by the Looms medical team, 2026-06-01.