Red Cell Distribution Width (RDW-CV)

Red blood cells are made in your bone marrow (the spongy tissue inside your large bones). They contain hemoglobin, an iron-rich protein that carries oxygen from your lungs to every cell in your body. Your cells need oxygen to grow, reproduce, and make energy for you to function. Normally, your red blood cells are about the same size. Differences in the size of your red blood cells may affect how well they can deliver oxygen through your body.

An RDW test uses a special graph called a histogram. If your red blood cells are about the same size, they will be close together on the histogram, and the RDW will be low. A high RDW means that the size of your red blood cells varies more than it should. The sizes will be more spread out on the histogram. This may be a sign of a medical condition.

RDW is most useful when looked at together with MCV (the average size of your red blood cells). For example, iron deficiency anemia tends to give small cells with a high RDW, while thalassemia (an inherited blood disorder) tends to give small cells with a normal or low RDW.

RDW results help your provider understand how much your red blood cells vary in size and volume. Results are usually shown as a percentage and compared to a reference range, which may vary depending on the lab used.

A normal RDW result means your red blood cells are within the reference range. Even if your RDW results are normal, your red blood cells may still be larger or smaller than usual. You may still have anemia or another medical condition that may need treatment.

A high RDW result means your red blood cells are higher than the reference range. This means that your red blood cells vary in size more than what is considered normal. This may be a sign of certain conditions, such as chronic liver disease, heart disease, diabetes, kidney disease, or cancer (especially colorectal cancer).

A low RDW result means your red blood cells are below the reference range. This means that they are similar in size and close together. This is not a sign of anemia and isn't usually something to worry about. Your provider will usually look at your RDW results along with the results of other blood tests to help diagnose iron deficiency, different types of anemia, thalassemia, and sickle cell anemia.