RDW (Red Cell Distribution Width)

Variation in red blood cell size — a marker of nutritional deficiency and stress.

What it is

RDW measures how much variation there is in red blood cell size. High RDW means cells vary more in size (anisocytosis). This occurs when the bone marrow is producing cells under nutritional stress or when multiple deficiencies coexist.

Why it matters

Elevated RDW is an early warning of iron, B12, or folate deficiency even before anaemia develops. It is also an independent prognostic marker in oncology — studies have linked high RDW to worse outcomes across multiple cancer types.

How to test

Full blood count.

This information is for educational purposes only. Always discuss testing and interpretation of results with your care team.

Optimal range

11.5–14.5%

normal

Above 14.5%

elevated — investigate B12, folate, iron

How often

Each blood count panel.