A measure of variation in red blood cell size.
RDW-SD, red cell distribution width by standard deviation, measures how much your red blood cells vary in size, reported as an actual width in femtolitres. A higher value means a wider mix of cell sizes.
It is calculated automatically by haematology analysers as part of the complete blood count. Unlike RDW-CV, which is a percentage, RDW-SD is a direct size measurement and is less affected by the average cell size.
A wide spread of red cell sizes, called anisocytosis, can be an early sign of certain anaemias. Iron deficiency, vitamin B12 or folate deficiency, and mixed deficiencies often raise RDW before other indices change.
Read with the mean cell volume, RDW helps sort out the cause of anaemia and can flag a developing problem even when haemoglobin is still normal.
Aniva reads your result against research-backed ranges, not just the lab's wide normal. The reference shown below is specific to this biomarker.
Typical adult guidance, analyser dependent:
| Result | Range |
|---|---|
| RDW-SD | 35 to 46 fL |
Ranges vary by analyser and laboratory. Read your result against your lab's reference interval.
Recent blood transfusion, a mix of deficiencies, and certain haemoglobin variants can affect the result. Clumped or aged samples reduce accuracy. Values differ between analyser models.
Read alongside haemoglobin, mean cell volume, ferritin, and vitamin B12 or folate when investigating anaemia.
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