What Does a High RDW Blood Test Result Mean?

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A high red cell distribution width (RDW) test means that red blood cells vary widely in size. Abnormal cell widths are often associated with anemia, liver disease and folate deficiency.

The red cell distribution width (RDW) test is usually performed as part of a complete blood count (CBC), which doctors order to understand the overall health of a person's blood. While the complete blood count measures many components of the bloodstream, the RDW test only evaluates the range of widths among red blood cells. It is often used along with the mean corpuscular volume (MCV) test to get a more accurate picture of red blood cell health. The MCV measures the volume of red blood cells in the body. These tests are usually ordered for patients with anemia, as they help doctors determine what kind of anemia a patient has.

Red Blood Cells

Red blood cells, also called erythrocytes, deliver oxygen from the lungs to various parts of the body. The normal size range for red blood cells is between six and eight micrometers per diameter. Expressed in percentage, which is how RDW is displayed, that equals 10.2 percent to 14.5 percent. Abnormalities in red blood cell volume and size can indicate a problem. Usually, red blood cell abnormalities come about from illness, vitamin deficiency or a genetic condition.

The RDW Test

Doctors might order a complete blood count and an RDW as part of a routine physical, or they might have a patient get tested if he or she exhibits signs of anemia, including weakness, fatigue and headache. Anemic patients might also have pale skin and cold hands or feet. Doctors may also order an RDW for patients with a family history of sickle cell anemia, genetic blood disorders or thalassemia. They will likely prescribe an RDW tests for patients with some chronic illnesses, including diabetes, HIV/AIDS and Chron's disease. Doctors might also order an RDW test for patients who have recently lost blood due to surgery and for those with chronic infections.

The RDW test is performed by a physician drawing a blood sample through a needle. The results are sent to a laboratory for analysis. Patients do not need to prepare for the test unless instructed by their physician. If they are getting other blood tests too, they may need to fast beforehand. The RDW is performed relatively quickly with few side effects, except for a slight risk of bruising at the site. As with any other blood test, patients should notify their doctors beforehand of any medications and supplements they are taking to ensure accurate test results.

What the Results Mean

Based on the results of the test, doctors can determine the underlying cause of the patient's high RDW levels. They will know if the problem stems from anemia, iron deficiency, thalassemia, liver disease and kidney disease. They might request additional testing if necessary to get more precise results, including determining what specific type of anemia a person has. Treatment depends on the underlying cause. Physicians might recommend dietary changes or supplements to improve a patient's health, or they can prescribe medication to alleviate symptoms. Doctors may recommend foods that are high in iron such as leafy green vegetables, seeds, nuts, avocados, bananas, mangoes, eggs and whole grains. Fortified cereals, dried beans and lentils are rich in folate.