Red Cell Distribution Width – SD (RDW-SD)
RDW-SD quantifies the spread of red cell sizes in femtoliters; elevated values indicate anisocytosis from nutritional or hemolytic causes.
Updated: 8/26/2026
Reference ranges (typical adult values)
| Who | Range | Unit |
|---|---|---|
| Adults | 37 – 54 | fL |
Ranges differ between laboratories — the range printed on your own report always applies first.
What is Red Cell Distribution Width – SD (RDW-SD)?
RDW-SD measures how much variation exists in the size of circulating red blood cells, expressed in femtoliters (fL). A higher RDW-SD means a wider spread of cell sizes; a lower value means more uniform cells.
While MCV tells you the average red cell size, it can remain deceptively normal when small and large cells average out. RDW-SD detects that hidden heterogeneity, making it a valuable partner to MCV in anemia evaluation.
Reference Range
37 – 54 fL
Reference ranges vary between laboratories and analyzer models.
Why Might It Be Elevated?
An elevated RDW-SD indicates anisocytosis — red cells of significantly different sizes:
- Iron deficiency anemia — one of the most common causes. Newer microcytes coexist with older normal-sized cells.
- Vitamin B12 or folate deficiency — megaloblastic anemia producing macrocytes alongside normal cells.
- Mixed nutritional deficiency — both microcytes and macrocytes, with a striking RDW-SD even when MCV appears normal.
- Hemolytic anemia — mix of large reticulocytes and surviving older cells.
- Post-hemorrhage — mixed-maturity cell populations.
- Myelodysplastic syndrome — as an early sign.
Why Might It Be Low?
- Thalassemia trait — uniformly small cells distinguish this from iron deficiency (where RDW-SD is elevated).
- Certain chronic anemias producing a homogeneous cell population.
What to Do Next
RDW-SD is a screening parameter, not a diagnosis. Your doctor will interpret it alongside MCV, hemoglobin, MCH, and reticulocyte count. Targeted tests (ferritin, B12, folate, blood smear) are typically ordered next.
Monitoring Over Time
RDW-SD changes slowly because red blood cells live approximately 120 days. After treatment, RDW-SD may temporarily increase further as new cells of different sizes enter circulation. Normalization typically takes 2–4 months. Monitoring every 2–3 months shows whether treatment is working.
Questions to Ask Your Doctor
1. What does my RDW-SD indicate together with my MCV and hemoglobin? 2. Should I have ferritin, vitamin B12, and folate tested? 3. If treatment starts, how long until RDW-SD normalizes? 4. Are there warning signs that should prompt earlier review?