Purpose, Preparation, Risks, & Results

The red cell distribution width (RDW) blood test measures the differences in the volume and size of your red blood cells. Red blood cells help carry oxygen from the lungs to other parts of your body. Changes in the size of your red blood cells often represent a sign of an underlying medical condition, such as anemia (a reduced number of red blood cells or hemoglobin).

Your healthcare provider may recommend an RDW test if they suspect you have an underlying condition. A high RDW means there is significant variation in the sizes of your red blood cells, which can be a sign of anemia, liver disease, or diabetes, among other conditions. A low RDW means there is a slight variation in the sizes of the red blood cells, which is not usually associated with any conditions.

The RDW blood test is sometimes a part of the complete blood count (CBC), a test that measures different parts of your blood, including:

• Red blood cells
• White blood cells
• Platelets
• Size and shape of your red blood cells

A CBC is part of routine check-ups and can help diagnose several disorders, such as anemia, infections, blood cancer, and autoimmune disorders. The RDW test, however, measures the difference in size and volume of red blood cells.

Normally, all red blood cells are about the same size, with a diameter of 6.2-8.2 micrometers (µm). Certain conditions, such as anemia, can lead to different sizes of your red blood cells. The RDW test can identify variations in red blood cell sizes and detect the underlying condition responsible for them. Your healthcare provider may recommend the RDW test if you have:

• Symptoms of anemia such as dizziness, weakness, and pale skin
• Iron or another vitamin deficiency
• Family history of sickle cell anemia, thalassemia, or any other inherited blood disorder
• Chronic conditions such as human immunodeficiency virus (HIV) or diabetes
• Significant blood loss from surgery or injury

You do not need any special preparation for the RDW test. Your healthcare provider might recommend other blood tests that may require fasting, so they may ask you to stop eating or drinking at least eight hours before you come in for your test. They will also inform you if you need to stop taking certain medications or supplements before the test.

The test often occurs at your healthcare provider’s office or a diagnostic clinic. You may want to wear short sleeves or loose-fitting clothes that make drawing blood from a vein in your arm easier.

Bring your insurance card and ID. If you’re not sure if your insurance will cover the cost of your test, consider calling your insurance company to find out about any out-of-pocket (OOP) costs.

The RDW test involves drawing blood from a vein in your arm. Once a healthcare provider collects your blood, the blood sample will be sent to a laboratory where an expert evaluates your blood cells under a microscope. They can learn whether there are variations in the size and volume of your red blood cells, which can help your provider understand if you’re living with an underlying condition.

During the Test

During the RDW test, your healthcare provider or a phlebotomist (a provider who specializes in blood tests) draws blood from a vein in your arm. You are often told to sit comfortably in a chair or sometimes lie down. The process of blood drawing includes the following steps:

• Locate a good-sized vein in your arm that is suitable for blood drawing
• Clean the area with antiseptic wipes
• Put an elastic band around the upper arm to put pressure on the area that will increase blood flow to the vein
• Insert the needle into the vein and collect blood into an airtight vial or tube attached to the needle
• Take the needle out and remove the band
• Put a clean gauze or bandage over the needle site to stop bleeding

The entire blood drawing process generally takes less than five minutes.

After the Test

In most cases, you can return home almost immediately and resume your daily activities after the test. However, your healthcare provider may ask you to stay back for up to 15 minutes if you experience dizziness, nausea, or extreme sweating after the test.

Your provider may also recommend that you avoid lifting heavy objects or performing strenuous exercise up to 24 hours after the test, as it may increase your risk of bleeding or bruising at the needle site.

The RDW blood test is relatively safe and carries minimal risks. However, you may experience slight pain or bruising at the needle site, which often disappears quickly. In very rare cases, you might develop some serious complications, such as:

• Severe pain at the needle site
• Nerve damage
• Leaking blood outside of blood vessels
• Infection
• Small red dots under the skin (known as petechiae)
• The formation of blood clots within a blood vessel (known as thrombosis)

If you experience side effects after the RDW test, it’s important to tell your healthcare provider immediately. They can help keep symptoms at bay and offer treatment if needed.

The RDW test results are available within a few hours to a few days after the test. In most cases, you’ll collect your test results from the clinic, a phone call from your provider, or through an online patient portal or app. If your provider is concerned about your results, they may set up an online or in-person follow-up appointment and let you know whether you need further testing.

The typical RDW test result ranges from 12-15%, which represents how many red blood cells in your blood sample deviate from the normal size. A test result higher than this percentage indicates “high” RDW levels, while a test result lower than this range is a sign of “low” RDW levels.

High RDW

A high RDW may be a sign of the following health conditions:

• Iron deficiency anemia
• Hemolytic anemia (when the immune system mistakes red blood cells as a threat and creates antibodies to destroy them)
• Vitamin B12 and folate deficiency
• Hereditary spherocytosis (a type of inherited blood disorder)
• Sideroblastic anemia (too much iron in the blood cells)
• Anemia associated with myelodysplastic syndrome (a type of cancer)
• Chronic liver disease
• Heart disease
• Diabetes
• Colorectal cancer (cancer of the colon or rectum)

Low RDW

Low RDW means there is very little variation in the size of the red blood cells. It is most often not associated with any medical conditions. However, if you’re experiencing symptoms and your results come back as “low,” your provider may recommend additional testing to understand what’s causing your symptoms.

Normal RDW

In some cases, you can have an underlying health condition even if your RDW results come back as normal. The following conditions can cause RDW levels to appear normal:

• Aplastic anemia: A condition where the bone marrow cannot make enough blood cells
• Thalassemia: An inherited blood disorder where the body does not make enough hemoglobin
• Anemia of chronic disease: Low red blood cells due to another health condition

An RDW test measures the difference between the volume and size of your red blood cells. Healthcare providers recommend this test if they suspect an underlying condition or if you have abnormal results on a complete blood count (CBC) test.

A low RDW result isn’t often associated with any conditions, while a high RDW result may be a sign of underlying conditions like anemia, liver disease, or diabetes.