RDW

What it is

RDW (red cell distribution width) measures how much your red blood cells vary in size — a low RDW means they're fairly uniform, a higher one means there's a wider mix of small and large cells. It's most useful paired with MCV: because MCV is an average, RDW is what reveals whether that average is hiding a lot of variation underneath. On its own RDW is quite non-specific — it can be nudged by many everyday and unrelated factors and by lab-to-lab differences in how it's calculated — so it's treated as supporting context within the whole CBC pattern rather than a signal by itself.

Typical range

Reference ranges vary between laboratories, and what is normal for you depends on your own history. Your own lab’s range is the one printed on your report.

Worth asking your clinician

• What does my RDW add when you look at it next to my MCV and hemoglobin?
• Has my RDW changed compared with previous CBCs, or is it steady?
• Is my RDW something worth following up on, or is it not meaningful given the rest of my results?