RDW (red cell distribution width)
RDW, red cell distribution width, shows how much red blood cells differ in size. It rises when old and new cells of different sizes mix in the blood, and it is often part of a complete blood count (CBC).
At a glance
- Sample
- Blood
- Units
- % (RDW-CV) or fL (RDW-SD)
- Often looked at with
- MCHC, Hemoglobin
- Preparation
- No special preparation is needed.
Units
Labs print RDW in two ways: RDW-CV as a percentage and RDW-SD in femtolitres (fL). They are two different measures of how red cell sizes spread, not two units of one number, so one can’t be converted into the other; some reports give both. Read each against the range printed next to it.
Why lab ranges differ
Each lab sets its reference range for its own method and analyser, and many ranges differ by age and sex. Two labs can print different ranges for the same blood, so a result is read against the range on its own report, not against a number from another lab or a website.
Soma charts every RDW result against the range printed by the lab that measured it.
What can affect the result
- A recent blood transfusion.
- A sample that sat too long before testing.
- Different analyzers: each lab’s range is for its own.
If something like this applies, mention it to the doctor who reads your results.
Questions
Why does my lab’s RDW range differ from another lab’s?
Each lab sets its range for its own method and equipment, and many ranges differ by age and sex. That’s why a result is best read against the range printed on the same report.
How do I prepare for an RDW test?
No special preparation is needed. Follow your lab’s instructions.
Related tests
Got a lab report with RDW?
Share it to Soma: it reads every row on your phone, with no account and no upload, and puts your RDW on one chart with your earlier results, each against the range your lab printed.
Sources
- RDW (Red Cell Distribution Width) — MedlinePlus, U.S. National Library of Medicine.
Mention of a source doesn’t mean it endorses Soma. This page explains the test; it doesn’t diagnose and is not medical advice. Talk to your doctor about what your results mean.