Red cell distribution width
What this test measures
The red cell distribution width describes how much the red blood cells vary in size. If all the cells are much the same size the RDW is low; if there is a mixture of small and large cells it is high. It is expressed as a percentage of the average cell size.
Why doctors request it
It is part of the full blood count and is read with the MCV. A high RDW with a low MCV is one of the pointers doctors use towards low iron rather than thalassaemia trait, and a high RDW can be an early sign that the red cells are changing.
What commonly pushes it up
- Low iron, especially early on or while it is being corrected
- Low vitamin B12 or folate
- A recent blood transfusion, which mixes two cell populations
- A high reticulocyte count, for example after blood loss
- Some bone marrow conditions
- Liver disease and alcohol
What commonly pushes it down
- There is no common cause of a low RDW; a low value is not usually of any importance
Everyday reasons for a value outside the range
- Analysers calculate it differently, so the same blood can read differently in two laboratories
- It rises a little with age in the general population
- It rises while low iron or a vitamin shortage is being corrected, which is expected
About the sample
No fasting needed. Read alongside MCV rather than on its own.
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Read my report freeThis is an educational summary of results you uploaded. It is not medical advice, a diagnosis or a treatment plan, and it does not replace the doctor who requested the test. If you are worried, or a result is marked outside range, discuss it with your doctor. Written and reviewed by a UK consultant physician. Reference ranges vary between laboratories.