RDW Test
Also known as: Red Cell Distribution Width, RDW-CV, RDW-SD
What is RDW?
RDW (Red Cell Distribution Width) measures the variation in size of your red blood cells. Higher values mean more variation in cell sizes.[MedlinePlus (NIH)]
What This Test Measures
This test measures how much red blood cell sizes vary from the average. It is expressed as a coefficient of variation (RDW-CV) or standard deviation (RDW-SD).[Cleveland Clinic]
Why It's Important
RDW helps differentiate between types of anemia and can indicate mixed nutritional deficiencies.[MedlinePlus (NIH)]
Elevated RDW has been associated with increased risk of cardiovascular disease and mortality.[Cleveland Clinic]
Who Should Get This Test?
RDW is routinely measured as part of a complete blood count, especially when evaluating anemia.
Risk Factors
- Symptoms of anemia (fatigue, weakness, shortness of breath)
- Recent blood loss or surgery
- Multiple nutritional deficiencies suspected
- Chronic inflammatory conditions
- Heart disease or heart failure
- Receiving chemotherapy
- History of blood transfusions
- Unexplained fatigue with normal hemoglobin
Screening Schedule
Part of routine CBC; particularly useful when distinguishing between different causes of anemia.
What Happens During the Test
A blood sample is drawn from a vein. RDW is calculated automatically by the hematology analyzer based on the spread of red blood cell volumes.
Duration
5-10 minutes for blood draw; results typically available same day
Discomfort Level
Minimal discomfort from needle insertion
Risks & Side Effects
Standard blood draw with minimal risks.
- •Brief pain at needle site
- •Small bruise may develop
- •Rare: lightheadedness
- •Very rare: infection at puncture site
Normal Range
RDW-CV: 11.5% to 14.5%. RDW-SD: 39 to 46 fL.
Note: Normal ranges may vary slightly between laboratories.
Source: MedlinePlus (NIH)Unit Conversion
Convert between % (RDW-CV) and fL (RDW-SD)
| % (RDW-CV) | fL (RDW-SD) |
|---|---|
| 12 | 42 |
| 13 | 45 |
| 14 | 49 |
| 15 | 52 |
| 16 | 56 |
High Values
High RDW indicates increased variation in red blood cell size (anisocytosis). Causes include iron deficiency, B12/folate deficiency, blood transfusion, and hemolytic anemia.[Cleveland Clinic]
Low Values
Low RDW is usually not clinically significant and indicates uniform red blood cell size.[MedlinePlus (NIH)]
Test Limitations & Accuracy
RDW interpretation has several considerations:
- ⚠RDW can be elevated temporarily after blood transfusion
- ⚠High reticulocyte counts increase RDW (young cells are larger)
- ⚠Cold agglutinins can affect red cell volume measurement
- ⚠Normal RDW does not rule out anemia
- ⚠RDW changes lag behind hemoglobin in response to treatment
- ⚠Some labs report RDW-CV (coefficient of variation) while others report RDW-SD (standard deviation)
- ⚠RDW is nonspecific and elevated in many conditions
Alternative & Complementary Tests
RDW is interpreted alongside other CBC components:
Combined with RDW to classify anemia type
To visually confirm variation in cell size (anisocytosis)
To assess bone marrow response to anemia
When iron deficiency anemia suspected
When macrocytic anemia with high RDW
Frequently Asked Questions
What does high RDW indicate?
High RDW means your red blood cells vary significantly in size. This can indicate iron deficiency, vitamin deficiency, recent blood loss or transfusion, or bone marrow disorders. It helps doctors identify the cause of anemia.
Source: Cleveland ClinicReferences
Related Condition Guides
Medical Disclaimer
This information is for educational purposes only and is not intended as medical advice. Always consult with a qualified healthcare provider for interpretation of test results and medical decisions.
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