Red cell distribution width (RDW)

Red cell distribution width (RDW)

May 3, 2022

Red cell distribution width (RDW)

What is red cell distribution width (RDW)?

The red cell distribution width (RDW) blood test measures the amount of red blood cell variation in volume and size.

Red blood cells carry oxygen from your lungs to every part of the body. Anything outside of the normal range in red blood cell width or volume indicates a possible problem with bodily function that in turn may affect oxygen getting to various parts of your body.

Normal red blood cells maintain a standard size of 6 to 8 micrometres (µm) in diameter. RDW is elevated if the range of sizes is large.

This means that if on average RBCs are small, but in the same sample there are also many very small cells, your RDW will be elevated. Similarly, if on average the RBCs are large, but in the sample, there are many very large cells, RDW will be elevated.

For this reason, RDW is not used as an isolated parameter when interpreting a complete blood count (CBC). Rather, it provides shades of meaning in the context of the haemoglobin (hgb) and mean corpuscular value (MCV).

High RDW values may mean you have a nutrient deficiency, anaemia, or other underlying condition.

Why is this analysis important?

The RDW test is used to help diagnose types of anaemia and other medical conditions including:

  • Thalassemias, which are inherited blood disorders that can cause severe anaemia

  • Diabetes mellitus

  • Heart disease

  • Liver disease

  • Cancer

What are some of the causes of high red cell distribution width (RDW)?

  • High RDW levels could be an indication of a nutrient deficiency, such as a deficiency of iron, folate, or vitamin B-12.

  • These results could also indicate macrocytic anaemia, when the body doesn’t produce enough normal red blood cells, and the cells it does produce are larger than normal. This can be due to a deficiency of folate or vitamin B-12.

  • Additionally, it can be caused by microcytic aneamia, which is a deficiency of normal red blood cells, and your red blood cells will be smaller than normal. Iron deficiency anaemia is a common cause of microcytic anaemia.

To help properly diagnose these conditions, your healthcare provider will perform a CBC test and compare the RDW and MCV test portions to measure your red blood cell volume.

A high MCV with a high RDW occurs in some macrocytic anaemias. A low MCV with a high RDW occurs in microcytic anaemias.

What are some of the causes of low red cell distribution width (RDW)?

There are no hematologic disorders associated with a low RDW result.