RDW (Red Cell Distribution Width)
A measurement of how much variation there is in the size of your red blood cells.
Last updated: April 10, 2026
RDW measures the range of sizes among your red blood cells. When all red blood cells are about the same size, the RDW is normal. When there is a mix of small and large cells, the RDW is elevated — a condition called anisocytosis. RDW is an important component of the complete blood count (CBC) panel and is especially useful for distinguishing between different types of anemia.
Why Is RDW Tested?
RDW is tested to help identify the type and cause of anemia and to distinguish between conditions that may look similar on other CBC markers. For example, both iron deficiency anemia and thalassemia trait can cause small red blood cells (low MCV), but RDW is typically elevated in iron deficiency and normal in thalassemia trait.
RDW is automatically calculated and reported as part of the CBC (complete blood count) panel. It is most clinically useful when interpreted alongside hemoglobin, hematocrit, and the mean corpuscular volume (MCV, which measures average red blood cell size).
Beyond anemia classification, recent research suggests that an elevated RDW may also be associated with cardiovascular disease risk, chronic inflammation, and overall mortality — though these associations are still being studied and RDW alone is not used to diagnose these conditions.
Normal Reference Ranges
Ranges may vary by lab, age, sex, and testing method. Always refer to the ranges printed on your specific lab report.
| Category | Range | Unit |
|---|---|---|
| Adults (RDW-CV) | 11.5 – 14.5 | % |
| Adults (RDW-SD) | 39 – 46 | fL |
| Children | 11.5 – 14.5 | % |
| Newborns | 14.5 – 18.5 | % |
Most labs report RDW-CV (coefficient of variation), which is the percentage form. Some labs also report RDW-SD (standard deviation), measured in femtoliters (fL).
| Status | RDW-CV (%) |
|---|---|
| Normal | 11.5 – 14.5 |
| Borderline High | 14.5 – 16.0 |
| High (Anisocytosis) | Above 16.0 |
What Does a High RDW Mean?
A high RDW means there is significant variation in red blood cell size, indicating the body is producing red blood cells of uneven sizes or that a mix of old and new cell populations is present.
Common causes:
- Iron deficiency anemia (the single most common cause of elevated RDW)
- Vitamin B12 deficiency (produces abnormally large red blood cells mixed with normal ones)
- Folate deficiency
- Mixed anemias (e.g., iron deficiency combined with B12 deficiency)
Less common causes:
- Myelodysplastic syndromes (bone marrow produces abnormal blood cells)
- Chronic liver disease
- Hemolytic anemia (premature destruction of red blood cells)
- Sickle cell disease
- Chronic inflammatory conditions
Temporary or benign causes:
- Recent blood transfusion (donor red blood cells mix with the recipient's cells, temporarily raising RDW)
- Recovery phase of anemia treatment (new, healthy cells mixing with older, abnormal cells)
- Newborns naturally have higher RDW values
An RDW of 14.5–16% is mildly elevated and often occurs with early nutritional deficiencies. Values above 18–20% suggest significant red blood cell production abnormalities and typically require further investigation.
What Does a Low RDW Mean?
A low RDW means the red blood cells are very uniform in size, which is generally considered normal and is not typically a cause for concern. Most clinical attention focuses on high RDW rather than low.
Conditions associated with normal or low RDW:
- Thalassemia trait (inherited condition with small but uniform red blood cells — low MCV with normal RDW)
- Anemia of chronic disease (red blood cells are uniformly small or normal-sized)
- Aplastic anemia (bone marrow produces fewer but normal-sized cells)
Important clinical note:
- A normal or low RDW does not mean the blood is healthy — it only means the red blood cells are similar in size. Other CBC values like hemoglobin and hematocrit must also be within range.
The distinction between iron deficiency (high RDW) and thalassemia trait (normal RDW) is one of the most clinically useful applications of the RDW test.
Factors That Can Affect Your Results
RDW is relatively stable compared to other CBC values, but some factors can influence the result.
- Recent blood transfusion: Mixing donor and recipient red blood cells of different sizes raises RDW
- Nutritional supplementation: Starting iron, B12, or folate supplements creates new cells of different sizes than existing ones, temporarily raising RDW
- Age: RDW tends to increase slightly with age, especially after 60
- Pregnancy: RDW may increase slightly due to iron demands and expanded blood production
- Alcohol use: Chronic heavy alcohol use can raise RDW due to folate deficiency and direct bone marrow toxicity
- Sample handling: Delayed processing of the blood sample may cause red blood cells to swell, potentially affecting results
- Medications: Chemotherapy and certain antiretroviral drugs can affect red blood cell size and raise RDW
Should I Be Concerned About My RDW Level?
RDW is most meaningful when interpreted alongside other CBC values, not in isolation. A slightly elevated RDW with normal hemoglobin, hematocrit, and MCV is often not clinically significant and may simply reflect normal variation or early nutritional changes.
If your RDW is elevated and your hemoglobin or hematocrit is also abnormal, your doctor will likely investigate further to determine the type of anemia. The combination of RDW with MCV (mean corpuscular volume) is particularly useful: high RDW with low MCV points toward iron deficiency, while high RDW with high MCV suggests B12 or folate deficiency.
If your RDW is significantly elevated (above 18%), it typically indicates a meaningful change in red blood cell production and warrants a conversation with your healthcare provider about potential causes and follow-up testing.
Common Next Steps
Follow-up testing is guided by the combination of RDW with other CBC values. A healthcare provider may recommend:
- Checking ferritin and iron studies to evaluate for iron deficiency
- Measuring vitamin B12 and folate levels
- Reviewing hemoglobin and hematocrit trends over time
- Ordering a reticulocyte count to assess bone marrow response
- A peripheral blood smear to visually examine red blood cell size and shape
- Hemoglobin electrophoresis if thalassemia is suspected
- Liver function tests if liver disease may be contributing
- Dietary assessment and potential supplementation for nutritional deficiencies
Frequently Asked Questions
What is a normal RDW level?
A normal RDW-CV is 11.5–14.5%. This means the red blood cells are fairly uniform in size. Values above 14.5% indicate increasing variation in cell size (anisocytosis). Newborns naturally have higher values (14.5–18.5%).
Should I worry about a slightly high RDW?
A slightly elevated RDW (14.5–16%) is common and may reflect early iron deficiency, B12 changes, or normal variation. If your hemoglobin and hematocrit are normal, your doctor may simply recommend monitoring and possibly checking iron or B12 levels.
Can iron supplements change my RDW?
Yes. When you start iron supplements for iron deficiency, the bone marrow begins producing new, normal-sized red blood cells while the older, smaller cells are still circulating. This mix of sizes temporarily raises RDW. As treatment continues and old cells are replaced, RDW gradually returns to normal over 2–3 months.
How does RDW help diagnose anemia?
RDW is used alongside MCV to classify anemia. Iron deficiency typically shows high RDW with low MCV. B12 or folate deficiency shows high RDW with high MCV. Thalassemia trait shows normal RDW with low MCV. This pattern recognition helps doctors narrow down the cause without needing extensive testing.
How often should RDW be retested?
RDW is retested whenever a repeat CBC is ordered. For borderline results, this may be in 4–8 weeks. If you are being treated for anemia, your doctor may repeat the CBC every 1–3 months. RDW changes slowly because it reflects the lifespan of red blood cells (about 120 days).
Do I need to prepare for an RDW test?
No special preparation is needed. Fasting is not required, and RDW is not significantly affected by food or hydration. The test is part of a standard blood draw for the CBC panel.
What is the difference between RDW-CV and RDW-SD?
RDW-CV (coefficient of variation) is a percentage that measures the relative variation in red blood cell size. Normal is 11.5–14.5%. RDW-SD (standard deviation) is an absolute measure in femtoliters. Normal is 39–46 fL. Most labs report RDW-CV, and most clinical references use the percentage form.
Can RDW indicate heart disease?
Research has found an association between elevated RDW and cardiovascular disease risk, including heart failure and stroke. However, RDW is not used as a standalone test for heart disease. The association likely reflects underlying inflammation or nutritional deficiencies that contribute to both elevated RDW and cardiovascular risk.
What is anisocytosis?
Anisocytosis is the medical term for red blood cells that vary significantly in size. It is reflected by an elevated RDW on a CBC report. Mild anisocytosis is common and may be noted as "slight" or "1+" on a blood smear. Moderate to severe anisocytosis suggests a more significant production problem and typically prompts further workup.
Sources
- Mayo Clinic. Complete blood count (CBC). mayoclinic.org
- MedlinePlus. RDW (Red Cell Distribution Width). U.S. National Library of Medicine. medlineplus.gov
- Cleveland Clinic. Red Cell Distribution Width (RDW) Test. clevelandclinic.org
- Salvagno GL, et al. Red blood cell distribution width: A simple parameter with multiple clinical applications. Crit Rev Clin Lab Sci. 2015;52(2):86-105
- Lippi G, et al. Red blood cell distribution width in cardiovascular risk assessment. Int J Lab Hematol. 2014
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Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges vary by laboratory, testing method, and individual factors. Always discuss your results with a qualified healthcare provider.