An RDW blood test measures your red blood cells’ variation in size and volume. It’s just one test that’s part of a more extensive blood workup called a complete blood count (CBC). An RDW blood test can help your healthcare provider diagnose anemia and other related conditions, along with other tests that provide information about your red blood cells.
An RDW (red blood cell distribution width) blood test measures how varied your red blood cells are in size and volume. Your red blood cells carry oxygen from your lungs to organs and tissues throughout your body. The oxygen fuels your body’s cells so that they can produce energy.
When it comes to your red blood cells, size matters. Healthy red blood cells are about the same size, ranging from 6.2 to 8.2 micrometers. Having red blood cells that are vastly different sizes from each other (high variation) may be a sign of anemia. Anemia is a condition that involves not having enough healthy red blood cells to supply oxygen to your body’s organs.
An RDW blood test is just one lab test your healthcare provider may use to diagnose anemia or other conditions.
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Your healthcare provider may order an RDW blood test as part of a more extensive blood workup called a complete blood count (CBC). A CBC is a blood test that provides information about multiple parts of your blood, including red blood cells, white blood cells and platelets.
One portion of a CBC includes a panel of tests called RBC (red blood cell) indices. RBC indices provide information about your red blood cells in particular. An RDW blood test is part of the panel.
RBC indices include:
Together, these tests may be used to diagnose various conditions associated with problems involving your red blood cells.
The word “width” in red cell distribution width can be confusing. It’s easy to assume that the test measures the width of your red blood cells, but this isn’t the case. Instead, an RDW blood test measures the variation in cell size among a representative sample of your red blood cells.
Cell distribution width refers to the distance between cell sizes as they’re plotted on a histogram. A histogram is a special type of graph that shows a range of numbers or values, like the range of red blood cell sizes. Similar values (more or less the same cell size) will be clustered close together on the histogram, and the RDW will be low. Different values (varied cell sizes) will be spread out on the histogram, showing a wider distribution of values. In this case, the RDW will be high.
An RDW blood test can help your healthcare provider get closer to a diagnosis if you have anemia-like symptoms or if you have a condition associated with anemia.
Your healthcare provider may suggest an RDW blood to find out if anemia is causing your symptoms. Common symptoms of anemia include:
An RDW blood test can also provide information on the specific type of anemia you have.
An RDW blood test, along with other RBC indices, may be used to help diagnose what’s causing anemia. Conditions associated with anemia include:
Having a long-lasting infection or losing a lot of blood during an accident or medical procedure may also cause anemia.
An RDW blood test involves a simple blood draw. Your blood sample is sent to a lab where a specialist examines your red blood cells under a microscope to check for size variations. The analyzed blood serves as a representative sample to better understand variations in red blood cell size throughout your body.
You don't need to do anything special to prepare for an RDW blood test. Your healthcare provider may order additional tests that require special preparation, like fasting, on the same day your blood is collected. In that case, follow their instructions to be sure you arrive fully prepared for whatever diagnostic tests you need.
The test should only take a few minutes. The process will be the same as a simple blood draw.
You should be able to leave shortly after the blood test. Your healthcare provider may recommend you wait a few minutes immediately after the blood draw to be sure you don’t experience side effects.
An RDW blood test is a safe procedure with little risk of side effects. You may notice slight bruising at the injection site, but the bruise should fade in a few days. You may feel lightheaded immediately after a blood draw, but this feeling usually doesn’t last long.
Let your healthcare provider know if the injection site doesn’t stop bleeding or if you begin feeling nauseated or faint. These side effects can happen, but they’re rare.
Test results appear as a percentage that lets you know whether your results are in the normal range or if your RDW is considered high or low.
A normal result means that your red blood cells are similar in size. Generally, a normal RDW ranges from 12% to 15%, but this number may vary depending on the lab that performs the test. Your results report will highlight what’s considered a normal range so that you can compare your numbers to what’s typical.
A normal result doesn’t mean that you don’t have a condition requiring treatment. Some types of anemia involve a normal RDW result but a high or low score on one of the other RBC indices.
A high RDW means that there’s variation in the size of your red blood cells beyond what’s considered normal. A high RDW may be a sign of anemia or a related condition. Your healthcare provider will need to look at other tests to be sure. Often, they’ll look at the results of an RDW (variation in red blood cell size) alongside the results of the MCV (average red blood cell size) to assess the health of your red blood cells.
A low RDW isn’t usually a cause for concern and isn’t associated with any particular types of anemia.
You’ll usually receive results within a few days after the lab receives your sample. Contact your healthcare provider if you need help interpreting the results. Reach out with questions about any next steps for diagnosing your condition.
A note from Cleveland Clinic
RDW blood test results that are high, low or even normal don’t provide enough information on their own for your healthcare provider to make a diagnosis. Still, an RDW blood test provides one piece of the puzzle when it comes to understanding how effectively your red blood cells are transporting oxygen throughout your body. Ask your healthcare provider about what your RDW blood test results mean in light of the other results from your CBC test.
Last reviewed by a Cleveland Clinic medical professional on 05/10/2022.
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