What it measures
RDW (red cell distribution width) shows how much red blood cells vary in size, as a percentage. The analyser works it out from the spread of red cell sizes it measures. A higher RDW means the cells are more mixed in size.
Why people test it
- Reported with many full blood counts, alongside the average red cell size (MCV).
- Used with MCV to help sort types of anaemia, such as low iron or low B12.
- Can rise early in iron deficiency, before other red cell results change.
Typical range
Ranges vary by lab and analyser, as machines calculate RDW in slightly different ways. Your report shows the lab's own range.
Finger-prick or vein?
A vein sample is usually advised. Blood from a finger prick can clot or break up (haemolyse) in the tube, which upsets blood-count results.