What is rdw in blood test
Last updated: April 1, 2026
Key Facts
- RDW is expressed as a percentage on CBC test results
- Normal RDW ranges from 11% to 15% in most clinical laboratories
- High RDW can indicate iron deficiency anemia, vitamin B12 deficiency, or folate deficiency
- RDW is used alongside other values like hemoglobin and MCV to diagnose blood disorders
- Abnormally high RDW may suggest chronic liver disease, malnutrition, or hemolytic anemia
What is RDW?
RDW stands for Red Cell Distribution Width, a measurement included in a complete blood count (CBC) test. It quantifies the variation in size of your red blood cells and is expressed as a percentage. Unlike other blood tests that measure absolute values, RDW measures the degree of variation or heterogeneity in red blood cell dimensions.
Understanding Normal Ranges
Most laboratories report normal RDW values between 11% and 15%. However, the exact reference range can vary slightly depending on the laboratory and the specific testing equipment used. Your healthcare provider will interpret your results based on the specific range provided by the laboratory that performed your test.
What High RDW Indicates
When RDW is elevated (higher than normal), it suggests that your red blood cells vary significantly in size. This condition, called anisocytosis, can indicate several underlying health issues:
- Iron deficiency anemia, one of the most common causes of high RDW
- Vitamin B12 deficiency or pernicious anemia
- Folate deficiency
- Chronic liver disease or alcoholism
- Hemolytic anemia or other blood cell disorders
RDW and Anemia Diagnosis
RDW is particularly useful for diagnosing different types of anemia. When combined with the Mean Corpuscular Volume (MCV), which measures average red blood cell size, RDW helps distinguish between various anemic conditions. For example, iron deficiency anemia typically shows both low hemoglobin and high RDW, while other conditions may show different patterns.
Interpreting Your Results
If your RDW is abnormal, your doctor will typically order additional tests to determine the underlying cause. These may include serum iron, ferritin, vitamin B12, and folate levels. It's important not to self-diagnose based on RDW alone, as it requires interpretation in context with other blood values and clinical symptoms.
Related Questions
What causes high RDW?
High RDW is most commonly caused by iron deficiency, vitamin B12 or folate deficiency, anemia, chronic liver disease, and malnutrition. These conditions create variation in red blood cell sizes that increases the RDW percentage.
What does a high RDW indicate?
High RDW indicates greater variation in red blood cell sizes and often suggests underlying medical conditions. Common causes include iron deficiency, vitamin B12 or folate deficiency, various types of anemia, chronic diseases, or alcohol use disorder.
What is MCV in a blood test?
MCV (Mean Corpuscular Volume) measures the average size of red blood cells. It's used alongside RDW to classify types of anemia and diagnose blood disorders more accurately.
Why would a doctor order an RDW test?
Doctors order RDW tests as part of routine health screenings or when patients show anemia symptoms like fatigue, shortness of breath, or weakness. RDW helps identify nutritional deficiencies and blood disorders while monitoring treatment effectiveness for chronic conditions.
Is high RDW serious?
High RDW alone isn't immediately serious, but it indicates an underlying condition needs investigation. The seriousness depends on the underlying cause, which your doctor can determine through additional testing.
How is RDW different from other blood cell measurements?
RDW specifically measures the variation in red blood cell size (distribution width), while hemoglobin measures oxygen-carrying capacity, hematocrit measures the percentage of blood that is RBCs, and MCV measures the average size of individual cells. Together they provide comprehensive blood health information.
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Sources
- Mayo Clinic - Complete Blood Count Fair Use
- CDC - Blood Basics Public Domain
- Wikipedia - Red Blood Cell Indices CC-BY-SA-4.0