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Red Blood Cell Distribution Width (RDW) - Whole Blood

Published: August 9, 2026Updated: August 17, 2026

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

Quick summary

["Your RDW level can help doctors check for conditions affecting your blood cells."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
47277-9
Short code
Erythrocyte DistWidth BldCo
Synonyms
Erythrocyte DistWidth BldCo
Category
chemistry
Sample
whole-blood
Container
Lavender-top (EDTA)
Units
N/A
Ref low
4.2
Ref high
5.9
Critical range
2 N/A – 7 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
85041
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 This Test Done?

Doctors may order an RDW test to investigate symptoms like fatigue, pale or yellowish skin, shortness of breath, or unusual bruising. It can also be used alongside other blood tests to diagnose or monitor conditions such as anemia, iron deficiency, or certain blood disorders.

What Does an RDW Test Measure?

The RDW measures the range of red blood cell sizes in your blood sample. Normally, RBCs are fairly uniform in size. If they vary widely, it may indicate problems like anemia, bone marrow disorders, or other health issues affecting RBC production.

How Is the RDW Test Performed?

A small amount of blood is drawn from a vein, usually in your arm. The blood is then analyzed in a lab to determine the RDW value. No special preparation is needed, but you may want to avoid strenuous activity or eating heavy meals before the test.

What Do RDW Results Mean?

Results are typically reported as a percentage. A higher RDW may suggest conditions like anemia (e.g., iron deficiency or vitamin B12 deficiency), thalassemia, or other blood disorders. A lower RDW might be normal or could indicate conditions like polycythemia vera. Your doctor will interpret results in the context of your overall health and symptoms.

Reference range

Adult males: 4.2-5.9; Females: 3.8-5.1 x 10^12/LN/A

Specimen: whole-blood

No special preparation usually required. Stay well hydrated. Inform your healthcare provider of all medications and supplements you are taking.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

RDW

Frequently asked questions

What Are Common Causes of an Elevated RDW?
Elevated RDW levels are often seen in conditions that affect red blood cell production or survival, such as anemia (including nutritional deficiencies like iron or vitamin B12 deficiency), chronic diseases, or certain cancers. It can also occur in response to infections or inflammatory processes.
What Are Common Causes of a Normal or Low RDW?
A normal or low RDW is generally not cause for concern unless other blood tests or symptoms suggest an underlying issue. It may be seen in healthy individuals or in conditions like polycythemia (too many red blood cells). Always discuss results with your healthcare provider.
How Often Should I Get an RDW Test?
Your doctor will determine how often you need an RDW test based on your medical history, symptoms, and other test results. Typically, it may be part of routine blood checks or monitoring for conditions like anemia. Follow your healthcare provider’s advice.
  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 17, 2026. · Read our editorial policy