Red Cell Distribution Width (SD)
Red cell distribution width (RDW-SD) measures the variation in your red blood cell size as a standard deviation, reported in femtoliters. It reflects the actual spread of cell sizes rather than a relative percentage.
RDW-SD is a direct measure of how wide the range of your red cell sizes is, and unlike RDW-CV it isn't influenced by the average cell size. This makes it a useful companion measure for characterizing anemias and mixed cell populations.
An above-range RDW-SD indicates a wider spread of cell sizes, which can accompany nutritional deficiencies (iron, B12, folate), recovering anemia, or a recent transfusion. Like RDW-CV, it's read alongside the other red cell indices.
A below-range or normal RDW-SD suggests your red cells are relatively uniform in size, which by itself is usually not concerning.
A typical RDW-SD runs roughly 39-46 femtoliters, though reference ranges vary by analyzer. Pair it with MCV and let your clinician interpret the combination.
Sources
- RDW (Red Cell Distribution Width) Test - MedlinePlus
- Red Cell Distribution Width (RDW) - Cleveland Clinic
Educational context only, not medical advice or a diagnosis. Reference ranges vary by lab, age and sex; always discuss your results with a clinician.