Red Cell Distribution Width
Also written as: RDW
How much your red blood cells vary in size from one another.
What it measures
The variation in size across your red blood cells. A high value means the cells are of noticeably mixed sizes rather than uniform.
Why a doctor orders it
Reported automatically with a CBC. It is most useful read together with MCV, because the combination narrows the possible causes of an anemia further than either does alone.
What is generally associated with a higher result
Increased variation is broadly associated with nutritional deficiencies including iron, B12 and folate, with recent blood transfusion, and with a body actively producing new red cells after blood loss.
What is generally associated with a lower result
A low or typical value simply indicates uniformly sized cells and is generally not pursued on its own.
Everyday things that move this number
Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:
- Recent transfusion
- Recovering from a treated deficiency
- Mixed deficiencies
Questions to take to your appointment
- How does this read alongside my MCV?
Build a printable questionnaire with your own results →
Where to read more
MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .
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