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Red Blood Cell Distribution Width (RDW)

Published: August 9, 2026Updated: August 16, 2026

This test measures the variation in size of your red blood cells (RBCs). Healthy RBCs are generally similar in size, but this test helps identify when they differ, which can signal underlying health issues.

Quick summary

["RDW helps doctors check for conditions affecting red blood cell size uniformity."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
14908-8
Short code
Rh Bld NB
Synonyms
Rh Bld NB
Category
chemistry
Sample
whole-blood
Container
Lavender-top (EDTA)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
3 mL
Stability
Room temperature: 4h, Refrigerated: 24h
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

Why is RDW Tested?

Doctors may order an RDW test to investigate symptoms like fatigue, pale skin, or unusual bruising. It’s also used alongside other blood tests to diagnose or monitor conditions such as anemia, iron deficiency, or vitamin deficiencies.

What Does an RDW Test Tell You?

A higher RDW suggests your RBCs vary widely in size, which can point to conditions like anemia (e.g., from iron or vitamin B12 deficiency) or chronic diseases. A lower RDW may indicate normal RBC production or certain blood disorders.

How Is the RDW Test Performed?

This test is done on a simple blood draw from a vein in your arm. A small amount of blood is collected and sent to a lab for analysis. The test measures the size variation of your red blood cells using automated technology.

What Are Common Causes of Abnormal RDW?

Elevated RDW is often linked to anemia (e.g., from low iron, folate, or vitamin B12), chronic diseases (like kidney disease or liver problems), or certain cancers. Low RDW is less common but may occur in rare blood disorders or after certain treatments.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance: RDW plays a key role in diagnosing and managing blood-related conditions. It can help distinguish between different types of anemia or guide treatment decisions, especially when combined with other blood tests.

Short name

RDW

Frequently asked questions

What does an abnormal RDW mean?
An abnormal RDW doesn’t give a direct diagnosis on its own, but it signals that your red blood cells may be producing inconsistently. Your doctor will review the results alongside other tests and symptoms to determine the next steps.
How often should I get an RDW test?
Your doctor will decide how often to check your RDW based on your health history, symptoms, and other test results. Some people may need it yearly, while others may need it less frequently or only when symptoms appear.
Can lifestyle changes affect RDW?
Yes, certain lifestyle factors like diet (especially iron, folate, or vitamin B12 intake), smoking, or chronic illnesses can influence RDW levels. Eating a balanced diet and avoiding smoking may help support healthy red blood cell production.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

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 16, 2026. · Read our editorial policy