Red cell distribution width: variation in red-cell size.
What it is
RDW quantifies anisocytosis. Reporting units vary by analyser (often %). Rising RDW means cells differ more in size.
Why it matters
A higher RDW means red cells vary more in size (anisocytosis). That pattern often appears in iron deficiency, mixed nutritional deficiency, or recovering marrow after treatment.
Read RDW with MCV, haemoglobin, ferritin, and B12/folate. Rising RDW can be an early hint that something in red-cell production is changing.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
How it affects the body
When the marrow releases cells of uneven size, for example early in iron therapy or with mixed deficiencies: RDW widens before average MCV fully normalises. It is a dispersion statistic, not a nutrient itself.
For athletes
Useful early hint that iron or vitamin status may be shifting across a heavy season. Follow up with proper iron studies and clinical advice, not guesswork supplements.
