Red blood cell distribution width (RDW) blood test measures the amount of red blood cell variation in volume and size. However, RDW is not used as an isolated parameter when interpreting a complete blood count (CBC).

What is an RDW blood test?

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

You need red blood cells to carry oxygen from your lungs to every part of your 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.

However, with certain diseases, you may still have a normal RDW.

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

This means that if on average your RBCs are small, but you also have a lot of very small cells, your RDW will be elevated. Similarly, if on average your RBCs are large, but you also have a lot of very large cells, your 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 hemoglobin (hgb) and mean corpuscular value (MCV).

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

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

This test is commonly performed as a part of a complete blood count (CBC).

The CBC determines the types and number of blood cells and various other characteristics of your blood, such as measurements of platelets, red blood cells, and white blood cells.

These tests help to determine your overall health status and, in some cases, diagnose infection or other diseases.

Doctors may also look at the RDW test as part of a CBC if you have:

Prior to an RDW blood test, you may be asked to fast, depending on what other blood tests your doctor has ordered. Your doctor will provide you with any special instructions before your test.

The test itself takes no more than 5 minutes. A healthcare provider will take a sample of your blood from a vein and store it in a tube.

Once the tube is filled the blood sample, the needle is removed, and pressure and a small bandage are applied over the entry site to help stop the bleeding. Your tube of blood will then be sent to a lab for testing.

If the needle site bleeding continues over several hours, visit a doctor immediately.

A normal range for red cell distribution width is 12.2 to 16.1 percent in adult females and 11.8 to 14.5 percent in adult males. If you score outside this range, you could have a nutrient deficiency, infection, or other disorder.

However, even at normal RDW levels, you may still have a medical condition.

To receive a proper diagnosis, your doctor must look at other blood tests — such as the mean corpuscular volume (MCV) test, which is also part of a CBC — to combine results and provide an accurate treatment recommendation.

In addition to helping confirm a diagnosis when combined with other tests, RDW results can help determine the type of anemia you might have.

High results

If your RDW is too high, it 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 anemia, when your 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, you may have microcytic anemia, which is deficiency of normal red blood cells, and your red blood cells will be smaller than normal. Iron deficiency anemia is a common cause of microcytic anemia.

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 anemias. A low MCV with a high RDW occurs in microcytic anemias.

Normal results

If you receive a normal RDW with a low MCV, you may have an anemia resulting from a chronic disease, such as that caused by chronic kidney disease.

If your RDW result is normal but you have a high MCV, you may have aplastic anemia. This is a blood disorder in which your bone marrow doesn’t produce enough blood cells, including red blood cells.

Low results

If your RDW islow, there are no hematologic disorders associated with a low RDW result.

Anemia is a treatable condition, but it can cause life-threatening complications if not properly diagnosed and treated.

An RDW blood test can help to confirm test results for blood disorders and other conditions when combined with other tests. Your doctor must reach a diagnosis before presenting you with treatment options, however.

Depending on the severity of your condition, your doctor may recommend vitamin supplements, medication, or dietary changes.

If you begin to experience any irregular symptoms after your RDW blood test or beginning treatment, call your doctor right away.