Red Blood Cell Distribution Width (RDW) Calculated from Mean Corpuscular Volume (MCV)
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 certain health conditions.
["Assessing RBC size consistency to detect potential blood disorders or nutritional deficiencies."]
Key facts at a glance
- LOINC
- 76138-7
- Short code
- RDWr Bld Calc-Rto
- Synonyms
- RDWr Bld Calc-Rto
- Category
- chemistry
- Sample
- whole-blood
- Container
- Lavender-top (EDTA)
- Units
- N/A
- Ref low
- 11.5
- Ref high
- 14.5
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 18–65 years
- Method
- calculated
- 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 RDW Matters in Your Blood Health
RDW is calculated using your MCV (average RBC size) and other blood measurements. While not a standalone diagnostic test, it often accompanies complete blood count (CBC) panels. Your doctor may order it to investigate symptoms like fatigue, anemia, or unexplained bleeding.
How RDW Works in Your Blood Test
Your blood sample is analyzed to determine the range of RBC sizes. If most RBCs are uniform, your RDW will be low. If there’s a wide spread in sizes, your RDW will be higher. This variation can reflect underlying issues like iron deficiency, vitamin B12 or folate deficiency, or even early stages of certain blood disorders.
Common Reasons for RDW Testing
Doctors may order RDW testing when you have symptoms like pale skin, shortness of breath, unusual fatigue, or unexplained bleeding. It’s also used to monitor conditions like anemia, pernicious anemia, or other blood-related disorders where RBC size consistency is important.
What to Expect Before or After the Test
No special preparation is needed for this test. A simple blood draw from your arm is all that’s required. Results are typically available within a few days, depending on your lab. Your doctor will discuss any follow-up steps based on your results.
Reference range
Adult: 11.5-14.5%N/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.
Clinical Significance: When RDW Points to Underlying Conditions
Short name
RDW (MCV-based)
Frequently asked questions
How long does it take to get RDW results?
Can RDW results be affected by diet or medications?
Is RDW testing painful?
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 16, 2026. · Read our editorial policy