Blood count
RDW
LOINC 788-0 · %
Ranges
Reference range: 11.5–14.5 %
What it is
RDW (red cell distribution width) measures how much your red blood cells vary in size.
Why it matters
More variation in cell size can be an early clue to nutrient deficiencies and helps sort out the type of anemia.
What high values can mean
A higher value means red cells differ more in size, often an early sign of iron, B12 or folate deficiency.
What low values can mean
A lower or normal value means red cells are fairly uniform in size.
How to improve it
Correcting an underlying iron or vitamin deficiency typically brings RDW back toward normal.
Food & lifestyle
Iron-rich foods plus adequate B12 and folate support uniform, healthy red cells.
Preparing for the test
Part of a complete blood count (CBC); fasting is not required.
When to talk to a doctor
A raised RDW is interpreted by a doctor with MCV and the rest of your CBC to find the cause.
Related markers
Sources
- MedlinePlus (NIH)
- Testing.com (AACC)
Content reviewed on June 26, 2026
Medical disclaimer
This content is for educational purposes only. It is not medical advice and does not diagnose, treat, or prevent any condition. Reference ranges vary between laboratories — always use the range printed on your report and discuss your results with a healthcare professional.
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