What it measures
White blood cells are the cells of the immune system that respond to infection. The test counts the total number in a litre of blood, and a full blood count usually breaks them down into types too, such as neutrophils and lymphocytes.
Why people test it
- Part of a full blood count, often checked during an infection or when someone feels unwell for a long time.
- Monitored on some prescriptions that can lower the count, such as certain thyroid, arthritis and cancer medicines.
- A raised count can follow infection, inflammation or some medicines, so it is read in context.
Typical range
Ranges vary between labs, with the method used, age and sex, and your report shows the lab's own range. One NHS trust lab gives 3.6 to 11.0 ×10⁹/L for adults. Source: Gloucestershire Hospitals NHS Foundation Trust: haematology reference ranges.
Finger-prick or vein?
A vein draw is usually advised. Finger-prick samples can clot in the tube, which upsets blood count results.