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Blood count and blood group · Blood
RDW
How much the red blood cells differ from each other in size; a supporting value when looking for the cause of anemia.
At a glance
- Measured in blood
- In 3 test panels
- No special preparation
Explanation
RDW: what is measured?
RDW (red cell distribution width) shows how much the size of the red blood cells differs from cell to cell, expressed as a percentage. Normally red blood cells are about the same size; the MCV is their average size. If the bone marrow makes new cells that are smaller or larger than the old ones, for example in early iron deficiency, the sizes spread out and the RDW rises. The value only has meaning together with the hemoglobin, the number of red blood cells and the MCV.
What can it detect?
- Early or existing iron deficiency, often before the MCV falls
- A mixed deficiency, for example iron together with vitamin B12 or folic acid, in which the MCV can look normal
- Recovery after blood loss, after a blood transfusion or after starting iron or vitamin tablets
Result
What does the result mean?
RDW too high
A raised RDW means that the red blood cells differ in size more than normal. The most common cause is iron deficiency: the new cells are smaller than the old ones. A deficiency of vitamin B12 or folic acid, a mixed deficiency, recovery after blood loss or after a blood transfusion, or the first weeks of treatment with iron or vitamins also raise the RDW. Less often it is an inherited abnormality of the red blood cells, such as spherocytosis (ball-shaped red blood cells). Rarely: a bone marrow disorder, almost always together with other abnormalities in the blood count.
RDW too low
A low RDW means that the red blood cells are very similar in size. This has no medical meaning and needs no follow-up. Many laboratories therefore only give an upper limit.
After an abnormal result
With a raised RDW, the GP looks at the hemoglobin and the MCV and, if needed, has ferritin, vitamin B12 and folic acid measured. A raised RDW with an otherwise normal blood count usually does not lead to further tests.
Reliability
How reliable is the test?
The RDW is calculated automatically by the blood cell counter and is not specific on its own: many different causes give a raised value. For finding the cause of anemia, the hemoglobin, the MCV and ferritin tell you more. A slightly raised RDW with a normal hemoglobin and MCV has little predictive value. The RDW is also used in calculation models for a 'biological age', such as PhenoAge; what such a score means for an individual result has not been established.
What can affect the result?
- Reference values differ by device and laboratory, and sometimes a different unit is used (RDW-SD in femtoliters).
- A blood transfusion, recent bleeding or starting iron, vitamin B12 or folic acid temporarily raises the RDW.
- Blood that is in transit to the laboratory for a long time makes the cells swell, which can affect the measurement of cell size.
- A normal RDW does not rule out iron deficiency; ferritin is the better test for that.
Who it is for
Who is the test useful for?
Useful for
Useful as part of the blood count with symptoms that fit anemia or iron deficiency, such as tiredness, shortness of breath on exertion and paleness, and when monitoring treatment with iron or vitamin B12. As a stand-alone value for healthy people without symptoms, the RDW adds little; a result within a 'biological age' calculation is also no reason for medical follow-up.
How common is it?
How often a raised RDW occurs is not known for the Netherlands. The main cause, anemia, is newly diagnosed in general practice in about 9 in 1000 people per year, in half of them due to iron deficiency, and in women about four times as often as in men.
NHG guideline on anemia (M76)
Drawbacks and risks
- A slightly raised RDW with a normal blood count usually means nothing, but can cause unnecessary worry or unnecessary further tests, especially if the value is interpreted on its own in a health or age score.
Get tested
RDW: test panels
RDW is part of these test panels. You order online and have your blood drawn at a collection point or collect the sample at home.
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Evidence
Sources
This explanation is based on Dutch guidelines and independent information. It is general information, not a diagnosis. Discuss an abnormal result with your GP.
More about risks and resultsMore explanations
Other tests: Blood count and blood group
- ABO blood group
- Basophils
- Eosinophils
- Erythrocytes (red blood cells)
- Hematocrit (Ht)
- Hemoglobin (Hb)
- Hemoglobin electrophoresis
- Leukocytes (white blood cells)
- Lymphocytes
- MCH
- MCHC
- MCV