RDW-SD
Red Cell Distribution Width - SD
- Sample
- Blood
- Usual units
- fL
Also known as Red Cell Distribution Width - SD · Red Cell Distribution Width SD · RDW SD
What this test measures
RDW-SD describes how much red blood cells vary in size, expressed as a width in femtolitres (fL). It is a different way of reporting size variation from RDW-CV, which is a percentage.
Reading your report
There isn’t one standard range for this test — each lab prints its own. Use the range shown next to your result.
Related guides
- Guide
Why the same blood test has different "normal" ranges at different labs
Reference intervals are built from each lab's own population and equipment, which is why two labs can print different ranges for the same sample.
- Guide
Reference interval, cutoff, and the "optimal" numbers online: three different things
The range beside your result, the target a website quotes, and the threshold that defines a condition are built in different ways and answer different questions.
- Guide
Why the same lab printed a different reference range for you this year
Ranges are not fixed constants. The lab can change method, revise its reference group, or apply a range that depends on your age — none of which is an error.
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