Red Cell Distribution Width.

Red cell distribution width also known as RDW.

This is a relatively new test and is not always included in your CBC. If it is not there do not worry. If anything in your normal CBC makes your doctor suspect a problem that he wants more confirmation on he can then order the RDW done.

It is often used together with MCV results when anaemia is suspected and they are trying to find the cause.

How it is done
The test can only be performed with modern instrumentation.
The instrument measures every cell in the sample and then works out the average size. It then compares each single red cell to that calculated average. It can thus tells the range of sizes in the blood sample.

Normal range
11-15%

How do they interpret the results?
A deficiency in certain substances can cause variation in cell sizes.

For instance Vitamin B12 and folic acid deficient cells are larger than iron deficient cells.

If the RDW is normal, with most of the cells being the same size, but the person is anaemic (based on other blood counts), then it is probably from a single cause, like Vit B12 deficiency, but if the RDW is increased then there is 'a mixed bag' of cells and the anaemia may have multiple causes such as both a Vit B12 and an iron deficiency.

Reticulocytes (immature red blood cells) are much larger than older cells and so anything that causes greater numbers of these to be released into the bloodstream will cause a rise in the RDW. This could be an indicator of severe blood loss or some disorder causing the premature destruction of cells, eg. Haemolytic anaemia.

What abnormal results mean

1. The RDW may be decreased due to:-
a) Iron deficiency anaemia caused by, for example -
Blood loss
Parasites
Malabsorption of iron
b) Vitamin B6 deficiency
c) Rheumatoid arthritis

2. The RDW may be increased in:-
a) Anaemias such as-
Pernicious anaemia - lack of Vit B12.
Folic acid anaemia
Iron deficiency anaemia combined with other anaemia
Haemolytic anaemia - a condition caused by a premature destruction of red blood cells.
Sideroblastic anaemia - a disorder where the body produces enough iron but is unable to put it into the haemoglobin.
Other hereditary anaemias eg sickle cell anaemia.
b) Transfusions
c) Alcohol abuse

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