Full Blood Count
What the most commonly requested blood test measures, and what the main abnormal patterns suggest.
Last revised
The most frequently requested blood test in the NHS. It counts and characterises red cells, white cells and platelets, and provides a broad screen across anaemia, infection, inflammation and bleeding disorders.
Red cells
Haemoglobin is the oxygen-carrying protein; low levels define anaemia. Mean cell volume describes red cell size and narrows the cause considerably: small cells point towards iron deficiency or thalassaemia, large cells towards B12 or folate deficiency, alcohol, thyroid disease or some medicines, and normal-sized cells towards chronic disease, kidney disease or acute blood loss.
White cells
The total count rises in many infections and inflammatory states. The differential matters more than the total: neutrophils typically rise with bacterial infection, lymphocytes with some viral infections, and eosinophils with allergy or parasitic infection. A very low neutrophil count leaves someone vulnerable to serious infection and is treated as urgent.
Platelets
Involved in clotting. Low counts increase bleeding and bruising risk; high counts can reflect inflammation, infection, iron deficiency or, less commonly, a bone marrow disorder. Both directions are interpreted alongside the rest of the count rather than alone.
How to read a result sensibly
Reference ranges are statistical, defined so that a proportion of healthy people fall outside them. A marginally abnormal value in someone well is frequently of no consequence, and repeating it later is often the appropriate response. Trends across several tests are considerably more informative than a single value.
At a glance
| Term | Detail |
|---|---|
| Also known as | FBC, or CBC outside the UK |
| Main components | red cells, white cells, platelets |
| Defines anaemia | low haemoglobin |
| Small red cells (low MCV) | suggests iron deficiency or thalassaemia |
| Large red cells (high MCV) | suggests B12 or folate deficiency, alcohol, thyroid disease |
| Fasting required | no |
| Typical turnaround | same day to a few days |
When to get help
Contact your GP promptly if a result is flagged as significantly abnormal. Seek urgent medical help for fever with a known low white cell count, particularly during chemotherapy, which is a medical emergency. Also seek urgent assessment for unexplained bruising, bleeding gums, tiny red spots in the skin that do not fade under pressure, or extreme breathlessness and pallor.
Where to go next
Where this comes from
- NHS - Blood tests
- NICE - Suspected cancer: recognition and referral
- Lab Tests Online UK - full blood count
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.
