RDW shows how much red blood cells vary in size (anisocytosis). The higher the RDW, the more "assorted" the cells.
It is a sensitive index: RDW often rises earlier than MCV changes and hemoglobin falls — so a high RDW can be an early signal of iron deficiency. In recent years RDW has also been studied as a marker of overall risk and aging.
Reference ranges
The reference range depends on the lab; typical values:
| Normal | 11.5 – 14.5% |
| High | assorted cells (anisocytosis) |
| Early sign | rises before hemoglobin falls |
| Helps with a normal MCV | distinguishes mixed anemias |
What an elevated level means
A high RDW means the red cells differ greatly in size.
Common causes: early or established iron deficiency, a deficiency of B12/folate, mixed anemias (when there are both small and large cells), a state after bleeding or treatment of anemia. A persistently high RDW is also associated with an increased risk of cardiovascular events.
What a low level means
A low RDW has no clinical significance — it means the red cells are uniform in size, and this is normal. Only an elevated RDW is diagnostically important.
How the test is done
RDW is calculated automatically in the complete blood count. It needs no special preparation and is interpreted together with MCV and hemoglobin.
A practical tip: a high RDW with normal hemoglobin is a reason to look at iron stores (ferritin) even before overt anemia develops.
What affects the value
- Iron deficiency — raises RDW (often early).
- B12 and folate deficiency — raises RDW.
- Mixed anemias — raise RDW noticeably.
- Recent bleeding or transfusion — raises it.
- Treatment of anemia — temporarily raises it (young cells appear).
- Chronic inflammation — can raise it.
Biological age from a blood test
RDW is part of the PhenoAge biological-age model — assess it:
Frequently asked questions
What does an elevated RDW mean?
That the red cells are of different sizes (anisocytosis). Most often this is early or overt iron deficiency, a B12/folate deficiency or a mixed anemia. RDW often rises before hemoglobin falls, so it serves as an early signal.
RDW is high but hemoglobin is normal — what to do?
This is a reason to check iron stores (ferritin) and, if needed, B12/folate: RDW can rise at the very earliest stage of a deficiency, before anemia develops.
Why is RDW linked to aging?
Large studies have shown that a persistently high RDW is associated with an increased risk of cardiovascular disease and all-cause mortality. So it is regarded as a non-specific marker of "biological trouble", though on its own it does not make a diagnosis.
Related terms
Sources
- RDW Test — Testing.com
- Red Cell Indices — Clinical Methods (NCBI)
- Levine M.E. et al. PhenoAge (uses RDW). NCBI/PMC