Red cell distribution width (RDW-SD)
Blood cell systemAlso known as: RDW standard deviation, red cell distribution width SD
Red Cell Distribution Width (Standard Deviation) is another measure of red blood cell size variation using standard deviation.
It provides similar information to RDW-CV but with a different calculation method.
Reference range
Female
Source: lab benchmark
Male
Source: lab benchmark
Reference ranges may vary between labs and assays. Always interpret results with your healthcare provider.
Why this matters
RD-SD (Red Cell Distribution Width – Standard Deviation) provides early insight into changes in red blood cell production before symptoms appear. Elevated values can indicate nutrient deficiencies or other factors affecting blood health. Monitoring this marker supports timely dietary or lifestyle adjustments to maintain healthy red blood cell function.
How this connects to other biomarkers
- RDW-SD is an absolute measure of red-cell size variation, less influenced by mean cell size than RDW-CV.
- Elevated RDW-SD with normal RDW-CV can suggest a population of larger cells emerging (early macrocytosis) before overall MCV rises — useful for catching early Vitamin B12 / Folic acid (Vitamin B9) deficiency.
- Interpret alongside MCV, Ferritin, B12, and folate.
How often should I test Red cell distribution width (RDW-SD)?
Most adults benefit from yearly RDW-SD testing as part of a complete blood count. When investigating anemia or supplementing iron, B12, or folate, retest at 3 months.
At baseline / for screening: Once every 12 months from age 30 as part of a comprehensive panel.
When monitoring an intervention or change: Retest 4 to 8 weeks after starting iron, B12, or folate supplementation for anemia, or after blood loss recovery. Red blood cells take about 120 days to fully turn over, so meaningful changes in cell size and content (MCV, MCH) take 3 to 4 months. Hydration on the morning of the draw shifts concentration-based measures, so reproduce conditions for reliable trends.
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