Red Blood Cell Distribution Width (RDW) on Whole Blood
This test measures the variation in size of your red blood cells (RBCs). Healthy RBCs are usually similar in size, but this test helps identify if there’s a wide range, which can signal certain blood conditions.
["RDW helps doctors check for anemia or other blood disorders by looking at how evenly sized your red blood cells are."]
Key facts at a glance
- LOINC
- 30385-9
- Short code
- Erythrocyte DistWidth Bld
- Synonyms
- Erythrocyte DistWidth Bld
- 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
What Does RDW Measure?
RDW (Red Blood Cell Distribution Width) evaluates the spread in size of your red blood cells. Normally, RBCs are fairly uniform in size. If they vary widely, it may indicate conditions like anemia (due to iron deficiency, vitamin B12 deficiency, or other causes), early stages of certain blood disorders, or even chronic diseases.
Why Might You Need This Test?
Your healthcare provider may order an RDW test to investigate symptoms like fatigue, pale skin, shortness of breath, or unusual bruising. It’s also used alongside other blood tests to diagnose or monitor conditions like anemia, iron deficiency, or pernicious anemia.
How Is the Test Performed?
A simple blood draw is taken from a vein, usually in your arm. The blood sample is sent to a lab where machines analyze the size and count of your red blood cells to calculate RDW. This test is quick and painless, similar to other routine blood tests.
What Are Common Causes of High or Low RDW?
High RDW levels may suggest conditions like anemia (from iron deficiency, vitamin deficiencies, or chronic diseases), early bone marrow disorders, or certain types of cancer. Low RDW is less common but could indicate healthy RBC production or specific genetic conditions. Always discuss results with your doctor.
Reference range
Adult males: 4.2-5.9; Females: 3.8-5.1 x 10^12/LN/A
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.
◆Associated findings1
Short name
RDW
Frequently asked questions
What does an abnormal RDW result mean?
How often should I get an RDW test?
Can lifestyle changes affect RDW levels?
Sources
- 1
- 2Lab Tests OnlineSource
- 3
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.
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