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RDW (Red Cell Distribution Width)
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

RDW (Red Cell Distribution Width)

The RDW test measures the variation in the size and volume of your red blood cells to help diagnose conditions like anemia and other blood disorders.

What is a red cell distribution width test?

A red cell distribution width (RDW) test measures how much the volume and size of your red blood cells (erythrocytes) varies. Normally, red blood cells are about the same size. Differences in size may affect how well they deliver oxygen throughout your body.

The test uses a graph called a histogram. If cells are uniform in size, the RDW is low. A high RDW means there is significant variation in cell size, which may indicate a medical condition.

What is it used for?

The RDW test is typically part of a complete blood count (CBC). It is commonly used to help diagnose or determine the cause of anemia, a condition where the body does not make enough red blood cells. It may also be used to help diagnose thalassemia, an inherited blood disorder.

Why do I need an RDW test?

Your provider may order this test as part of a routine exam or if you exhibit symptoms of anemia, such as:

  • Shortness of breath
  • Weakness or fatigue
  • Headache
  • Dizziness
  • Arrhythmia (irregular heartbeat)

Other reasons for testing include a family history of inherited blood disorders, chronic illnesses (such as Crohn's disease, diabetes, or HIV), a diet low in iron, or excessive blood loss.

What happens during the test?

A health care professional will take a blood sample from a vein in your arm. The procedure is quick, typically taking less than five minutes. No special preparation is necessary.

What do the results mean?

  • Normal RDW: Your red blood cells are within the reference range. Note that you may still have anemia or another condition even with a normal RDW.
  • High RDW: Your red blood cells vary in size more than normal. This may be a sign of chronic liver disease, heart disease, diabetes, kidney disease, or cancer (especially colorectal cancer).
  • Low RDW: Your red blood cells are similar in size. This is generally not a sign of anemia and is not typically a cause for concern.

Providers usually evaluate RDW results alongside other blood tests to diagnose specific conditions like iron deficiency, thalassemia, or sickle cell anemia.

Frequently Asked Questions

high RDW suggests a mix of small and large red cells, common in iron deficiency, vitamin B12/folate deficiency, or thalassemia. Knowing the cause helps target therapy—iron supplements for iron deficiency, B12 injections for deficiency, or further genetic testing for thalassemia.
Yes. Early iron deficiency often shows a normal RDW because red cells are uniformly small. As the deficiency progresses, RDW rises. So a normal RDW does not rule out iron deficiency, especially if ferritin is low.
Your provider may order serum ferritin, transferrin saturation, vitamin B12, folate levels, hemoglobin electrophoresis, and a reticulocyte count. These help differentiate between iron deficiency, B12/folate deficiency, thalassemia, or other marrow disorders.===
Increasing intake of iron-rich foods (red meat, beans, fortified cereals), vitamin C (to enhance absorption), and ensuring adequate B12/folate (leafy greens, eggs, dairy) can improve red cell uniformity. Supplements should be taken under a clinician’s guidance.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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