RDW Blood Test

Red blood cell distribution width (RDW) measures variations or differences in the size and volume of your red blood cells (RBC), the cells that carry oxygen throughout your body. An RDW measurement is part of a complete blood count (CBC) test.

The term distribution width refers to the distance between cell sizes as they’re plotted on a histogram, a graph that shows the range of red blood cell sizes.

Size matters when it comes to red blood cells. Healthy red blood cells are about the same size, ranging from 6.2 to 8.2 micrometers. A graph of healthy red blood cells would show clusters of cells that are about the same size. But the graph may show cells scattered in various places. This means your red blood cells are of different sizes.

Red blood cells that are of very different sizes may be a sign of anemia. Healthcare providers use RDW results to diagnose anemia and conditions that cause anemia.

How does an RDW test work?

An RDW measurement checks whether red blood cells are about the same size. A healthcare provider sends your blood sample to a lab where a specialist examines your red blood cells under a microscope.

What to expect during the test

The test to get blood for RDW analysis takes a few minutes. Your healthcare provider will:

1. Pick the best spot to draw blood, usually in the crook of your arm or on the back of your hand.
2. Clean and disinfect the area where they’ll insert a needle into your arm or hand.
3. Place an elastic band on your arm above the spot where they’ll insert the needle. This slows down your blood flow, which makes it easier for your provider to see and access.
4. Insert the needle to take the blood sample. You may feel a little sting or a pinch when the needle goes into your arm. The needle connects to a vial that fills with blood.
5. Remove the needle after collecting blood. Your provider will place a bandage on the spot where they inserted it.

Does this test cause side effects?

Blood tests are safe procedures with little risk of side effects. You may feel lightheaded immediately after a provider draws blood. This feeling usually goes away within a few minutes. Your provider may recommend you delay leaving until that feeling goes away.

Later, you may notice slight bruises where your provider placed the needle. Any bruises should fade in a few days.

What type of results do you get and what do they mean?

Red blood cell distribution width results appear as a percentage that shows if your RDW is high, normal or low:

• High RDW: In this case, the sample shows 15% or more of your red blood cells are of different sizes. A high RDW may be a sign of anemia or a related condition. Your healthcare provider will do other tests to diagnose the cause. For example, they may compare RDW results with mean corpuscular volume (MCV) test results. This test measures the average size of your red blood cells.
• Normal RDW: Your result will show the percentage of red blood cells in the sample that are similar in size. A normal RDW ranges from 12% to 15%. But the range may vary depending on the lab that analyzed your blood. A normal RDW result doesn’t rule out the possibility that you have another condition. For example, you may have a type of anemia that an RDW test doesn’t detect, but other RBC tests do. 
• Low RDW: The result shows your RDW range is lower than 12%. This result isn’t a sign of an underlying condition or anemia.

When will I get my test results?

You and your healthcare provider will probably receive your results within a few days. Your provider may schedule an appointment to discuss your results. But don’t hesitate to contact them if you have questions about your results.

What are the next steps if tests show high RDW?

This result may be a sign of anemia or a related condition. Your healthcare provider will explain what your results mean. They’ll also discuss any other tests they may do. For example, they may compare RDW results with MCV test results.

When should I call my doctor?

Contact your healthcare provider if your anemia symptoms worsen. Some common symptoms are:

• Fatigue
• Pallor
• Shortness of breath
• Irregular heartbeat

What other tests check on red blood cells?

A complete blood count test includes a group of tests called red blood cell indices. Healthcare providers use red blood cell indices to check for issues with these cells. An RDW test is in this test group. Other tests are:

• Mean corpuscular volume (MCV): This test measures the average size of your red blood cells.
• Mean corpuscular hemoglobin (MCH): An MCH test measures the average amount of hemoglobin in your red blood cells. Hemoglobin is a protein that allows your RBCs to transport oxygen throughout your body.
• Mean corpuscular hemoglobin concentration (MCHC): This test measures the average amount of hemoglobin that’s concentrated (close together) in your red blood cells.

Does this test diagnose other conditions?

Several conditions can cause anemia. Providers may use a RDW test result, along with other RBC indices, to help diagnose the underlying condition. Conditions associated with anemia include:

• Cancer
• Cardiovascular disease
• Kidney disease
• Liver disease
• Thalassemia
• Chronic illnesses like Crohn’s disease or HIV
• Mineral and vitamin deficiencies like low levels of folate, iron or vitamin B12

Red blood cell tests are one piece of the diagnostic puzzle. Your provider will do several different tests, like imaging tests or biopsies, to diagnose the conditions that cause anemia.