What laboratory test provides the red cell distribution width (RDW)?

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What Laboratory Test Provides RDW?

Red cell distribution width (RDW) is automatically calculated and reported as part of the complete blood count (CBC) performed by automated hematology analyzers. 1

How RDW is Obtained

  • RDW is included in every routine CBC without requiring a separate order or additional blood sample 1
  • The parameter is automatically calculated by the analyzer from the standard deviation of red blood cell volumes 1
  • No special preparation or additional testing is needed beyond ordering a standard CBC 1

Technical Calculation

  • The Centers for Disease Control and Prevention (CDC) defines RDW-CV (coefficient of variation) as the standard deviation of red blood cell volume divided by the mean corpuscular volume (MCV), multiplied by 100 to express as a percentage 1
  • The formula is: RDW (%) = {SD of red blood cell volume (fL)/MCV (fL)} × 100 1
  • Some analyzers also report RDW-SD, which represents the absolute standard deviation of red blood cell volumes measured in femtoliters 1

Normal Reference Range

  • Normal RDW is generally ≤14.0%, corresponding to the 95th percentile in reference populations 1
  • The typical reference range is 11.0-14.0% 1
  • Values should be interpreted according to your specific laboratory's reference range, as different automated counters may yield slightly different RDW values 1, 2

Clinical Context

  • RDW quantifies heterogeneity in the size of circulating erythrocytes (anisocytosis) 3
  • It is traditionally used for differential diagnosis of anemias but has broader clinical applications in cardiovascular disease, thromboembolism, and mortality prediction 4
  • RDW should always be interpreted alongside other CBC parameters including hemoglobin, MCV, and other red cell indices 1

Important Caveat

  • Different electronic counters yield different RDW values, so laboratories must verify their own reference ranges 2
  • RDW serves best as a guide in differential diagnosis rather than a definitive test by itself 2

References

1
Guideline

Interpreting Red Cell Distribution Width (RDW) Values

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2026

3
Research

Association of the red cell distribution width with red blood cell deformability.

Advances in experimental medicine and biology, 2013

4
Research

Red blood cell distribution width: A simple parameter with multiple clinical applications.

Critical reviews in clinical laboratory sciences, 2015

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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