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Overview

What Is the RDW Blood Test?

The RDW blood test measures red cell distribution width, which describes how much red blood cells vary in size. It is usually reported as part of a complete blood count (CBC), alongside measurements such as hemoglobin and mean corpuscular volume (MCV), rather than interpreted by itself. This blood analysis can help clinicians recognize patterns that may need further evaluation.

When It Is Needed

When Is an RDW Blood Test Needed?

A clinician may order a CBC with RDW when symptoms or other findings suggest a blood-related problem. Common reasons include evaluating possible anemia or a nutritional deficiency, investigating unexplained symptoms, or monitoring a known blood disorder. An RDW may also be included in routine health testing, but an abnormal result does not automatically mean that a disease is present.

  • Fatigue, weakness, dizziness, or reduced exercise tolerance may prompt testing for anemia.
  • Pale skin or shortness of breath may lead a clinician to check red blood cell measurements.
  • Suspected iron, vitamin B12, or folate deficiency may be evaluated with a CBC and other tests.
  • A known anemia or blood disorder may require follow-up testing to monitor changes.
  • A CBC may be ordered as part of a broader health evaluation even when no specific symptoms are present.

Preparation

How to Prepare for an RDW Blood Test

An RDW blood test generally requires no special preparation. If the same blood draw includes tests that require fasting, your clinician or laboratory will give you separate instructions, so follow the guidance provided for the full order. Do not change your usual medicines unless a healthcare professional specifically tells you to do so.

During the Test

What Happens During the RDW Blood Test?

An RDW blood test is a brief, single-session venous blood draw. A tourniquet is placed around your arm, the skin is cleaned, and blood is collected into a tube through a small needle. The needle is removed and the site is covered with a bandage before the sample is sent to the laboratory for analysis.

How the laboratory reports RDW

The laboratory may report RDW-CV, which is usually shown as a percentage, and/or RDW-SD, which is an absolute measurement. The format depends on the laboratory analyzer and reporting system. You may feel a brief pinch or pressure, and minor bruising can occur; most people can return to normal activities unless other tests or personal circumstances require different instructions.

Risks

Are There Risks to the RDW Blood Test?

  • A brief pinch, soreness, or pressure can occur when the needle enters or leaves the vein.
  • Minor bruising or slight bleeding may occur at the puncture site.
  • Some people feel lightheaded or faint during or shortly after the draw.
  • Infection is uncommon but can occur at the puncture site.
  • Occasionally, the area may remain tender for a short time.

Seek advice

Problems after the blood draw

Contact a healthcare professional if bleeding does not stop with firm pressure, or if you develop increasing swelling, severe pain, spreading redness, warmth, drainage, or other signs of infection at the puncture site.

Results

Understanding RDW Blood Test Results

RDW results may be reported as RDW-CV, a percentage, and/or RDW-SD, an absolute measurement. Use the reference range printed on your laboratory report because methods and ranges can vary between laboratories. A result that is high or low should be considered with the rest of the CBC rather than interpreted as a diagnosis.

RDW resultGeneral meaningInterpretation context
Within reference rangeLess variation in red blood cell size is reportedInterpret with hemoglobin, MCV, and the rest of the CBC
Higher than reference rangeMore variation in red blood cell size is reportedMay occur with several types of anemia or other conditions and needs clinical context
Lower than reference rangeRed blood cells are relatively similar in sizeUsually is not interpreted alone and should be reviewed with the complete blood count

Why other blood tests matter

Clinicians may compare RDW with MCV, hemoglobin, ferritin or iron studies, reticulocyte results, and other CBC findings to understand a pattern. Platelet measures such as MPV may provide additional information, but they measure a different blood cell feature. NRBC is also a separate measurement, so an NRBC blood test result should not be confused with RDW. Laboratory processing may take some time; discuss a persistent abnormality, symptoms, or questions about a high RDW or low RDW with the clinician who ordered the test instead of self-diagnosing.

Diseases & Conditions

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