Lab test glossary · Blood count
RDW: what it means
Also known as: red cell distribution width
RDW measures how much your red blood cells vary in size; a high RDW can be an early clue to a developing or mixed anemia.
What it measures
RDW (red cell distribution width) describes how uniform your red blood cells are in size. When the body starts producing cells of differing sizes — for instance, early in an iron or B12 deficiency — RDW rises, sometimes before other numbers budge. It's most useful read alongside MCV.
Typical range
Often roughly 11.5–14.5%.
Reference ranges differ between labs and are printed on your own report — always read your result against that range, not a general one.
What a high result can mean
- Iron, B12, or folate deficiency (often early)
- A mix of two causes of anemia at once
- Recent blood loss or transfusion
A high value is a starting point for a conversation, not a diagnosis — many causes are benign or temporary.
Why the trend matters
A rising RDW is a classic early-warning number, so its trend can flag a developing deficiency before an anemia is obvious.
When to talk to your doctor
Discuss a high RDW with your doctor in the context of your MCV and hemoglobin.
This page is general education about a common lab test — it isn’t medical advice and can’t interpret your specific result. What a value means depends on your history, symptoms, and the rest of your labs, which is a conversation for you and your clinician.