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Red Cell Distribution Width (RDW)

Published: July 27, 2026Updated: August 25, 2026

RDW measures the variation in the size of red blood cells (RBCs) and is expressed as a percentage. It helps in diagnosing various types of anemia and other blood disorders.

Quick summary

RDW assesses the heterogeneity in red blood cell size.

Medically reviewed by WellAlly Editorial TeamLast reviewed August 25, 2026Editorial policy

Key facts at a glance

Units
%
Clinical utility
diagnostic
Result kind
semi_quantitative
FDA-cleared
Sysmex RDW, Abbott RDW, Siemens RDW
interpretation: {"text":"Indicates more uniform RBC size, typically normal or may suggest certain blood disorders if other parameters are abnormal.","label":"Low RDW"}; {"text":"Suggests increased variation in RBC size, which may be due to anemia, nutritional deficiencies, or other blood disorders.","label":"High RDW"}

RDW is a measure of the size variability of red blood cells (RBCs) in a blood sample. It is calculated by dividing the standard deviation of RBC size by the mean corpuscular volume (MCV) and multiplying by 100 to get a percentage.

A higher RDW value indicates greater variation in RBC size, which can be a sign of anemia, nutritional deficiencies, or other blood disorders. Conversely, a lower RDW suggests more uniform RBC size.

RDW is often used in conjunction with other hematological parameters, such as mean corpuscular volume (MCV), to provide a more comprehensive assessment of red blood cell morphology and to aid in the diagnosis of conditions like iron deficiency anemia, vitamin B12 deficiency, and thalassemia.

Frequently asked questions

What is a normal RDW range?
The normal RDW range is typically 11.5–14.5%.
Can RDW be used to diagnose a specific condition?
RDW alone cannot diagnose a specific condition but is useful in conjunction with other tests to identify various types of anemia and blood disorders.
  1. 1
    Mayo Clinic: RDW Blood TestSource
  2. 2
    MedlinePlus: RDWSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 25, 2026. · Read our editorial policy