Red Cell Distribution Width (RDW-CV)
How much your red blood cells vary in size. A higher number means a wider mix of small and large cells.
AI-verified against cited sources. Awaiting expert reviewAlso called RDW-CV, RDW, RDW CVLOINC 788-0
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What it is
RDW-CV stands for red cell distribution width, coefficient of variation. The blood analyzer works it out from the sizes of your red blood cells and reports it as a percentage. If most cells are about the same size, the number is low. If there is a wide mix of small and large cells, it is higher. It comes with a complete blood count (CBC) and is read alongside MCV, the average cell size.
Why it matters
RDW is an early clue when sorting out anemia and nutrient shortages. It often rises when the body is short on iron, vitamin B12 or folate, or when red cells are being made or lost at different rates. It can also be a little high for reasons that have nothing to do with anemia. In large studies of adults, a higher RDW is also linked to higher health risks over many years. That is a pattern across groups, not a prediction for any one person.
If it’s high
A high RDW means your red cells vary more in size than usual. The most common reasons are low iron, low vitamin B12 or folate, or a recent blood loss or transfusion. Many other conditions can raise it too, including long-term illness, liver or kidney disease and inflammation. Your clinician reads it together with MCV, hemoglobin and your other results.
Common causes
- Low iron
- Low vitamin B12 or folate
- Recent blood loss or a recent blood transfusion
- Inherited blood conditions such as thalassemia or sickle cell disease
- Long-term illness or inflammation
- Liver, kidney, heart or lung disease
- Diabetes
- Some medicines, including chemotherapy
- Heavy alcohol use
What can help
- Ask your clinician whether iron, B12 and folate levels should be checked
- Ask whether MCV, hemoglobin or a repeat test would help explain the pattern
- Eat a balanced diet that includes iron, B12 and folate foods, and keep up with routine checkups
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 11.7–15.4 %. Source: Labcorp, American Board of Pathology, Te Whatu Ora Health New Zealand, Waikato, Archives of Internal Medicine, The Journals of Gerontology: Series A.
- Adults (this one applies)11.7–15.4 %
Sources: Labcorp, American Board of Pathology, Te Whatu Ora Health New Zealand, Waikato, Archives of Internal Medicine, The Journals of Gerontology: Series A
See RDW by age and sex, with the U.S. survey numbers where they exist.
How common is your level?
NHANES describes the US civilian, non-institutionalized population, not a 'healthy' reference group. People with disease, on medication, fasting or not (except fasting-subsample markers), smokers and so on are all included. A percentile says where a value sits among Americans of that sex and age, not whether it is normal.
Use the left and right arrow keys to read each age. Shift moves five years.
Reference data derived from CDC/NCHS National Health and Nutrition Examination Survey (NHANES), cycles 2015-2016, 2017-March 2020 (pre-pandemic), August 2021-August 2023. Analysis by bloodtesttracker.com; not endorsed by CDC, NCHS or HHS.
Explore RDW by age and sex: every percentile as a table, with a download.
What can affect your result
- The analyzer and method: limits differ between labs
- Recent blood transfusion, which mixes cells of different sizes and ages
- Recent blood loss
- Iron, vitamin B12 and folate status
- Age: RDW can rise slightly with age
- Pregnancy
- How long the blood sat before testing
- A recent blood transfusion mixes red cells of different sizes and can raise RDW.
- Cold agglutinins and red cell clumping can distort the size measurements.
- RDW-CV depends on the analyzer, so results from different labs or machines are not directly comparable.
- Blood that sits for many hours before testing can change red cell size measurements slightly.
Changes over weeks to months, because it follows red cell turnover (about 120 days).
When to talk to a clinician
There is no urgent-call limit for RDW. Contact a clinician if a high RDW comes with tiredness, shortness of breath, pale skin, a fast heartbeat or unexplained weight loss. Call 911 or your local emergency number for chest pain, severe shortness of breath, fainting or confusion.
Questions you could ask
- Has my RDW changed compared with my earlier results?
- Could low iron, B12 or folate explain my RDW?
- Should we look at my MCV and hemoglobin together, or repeat the test?
Questions people ask
My RDW is a bit high but my hemoglobin is in the usual range. What does that mean?
That is a common result. A slightly high RDW can be an early sign that iron, B12 or folate is running low before anemia shows up. It can also come from many other things, including a recent illness or the analyzer your lab uses. Your clinician can decide whether more tests would help.
What is the difference between RDW-CV and RDW-SD?
Both measure how much red cell size varies. RDW-CV is a percentage and is tied to the average cell size (MCV). RDW-SD is measured in femtoliters (fL) and is a direct width. Most US labs print RDW-CV. The two have different ranges.
I read that RDW is linked to longevity. Should I worry?
In large studies of adults, people with a higher RDW were more likely to die over the following years, even after accounting for age and other factors. That is a pattern across big groups, and nobody knows whether RDW is a cause or a sign of other health problems. One result can't predict anything for you. If yours is high, a clinician can check for common, fixable causes such as low iron or B12.
Can I lower my RDW?
You can't change RDW directly. If it is high because of low iron, B12 or folate, treating that with your clinician's guidance can bring it down over weeks to months, because red cells turn over about every 120 days. A balanced diet and good care of long-term health conditions support healthy red cells.
Show the science and sources
Is a change real?
RDW naturally varies about 3.5% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 11% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Desirable Biological Variation Database specifications, 2014 update (Minchinela J, Ricos C, et al.; the 'Ricos database', a compilation of median published CVI/CVG).
For clinicians and researchers
RDW-CV is the standard deviation of red cell volume divided by the mean (MCV), times 100. It is calculated from the red cell volume histogram, so it depends partly on MCV and on the analyzer. RDW-SD is a direct width measure in fL and depends less on MCV. Classic patterns: a high RDW with a low MCV is typical of iron deficiency, while thalassemia trait often has a normal RDW with a low MCV. A high RDW with a high MCV fits B12 or folate deficiency. RDW has also been studied as a prognostic marker. In NHANES III (15,852 adults), each 1-SD increase in RDW (0.98%) went with an adjusted hazard ratio of 1.23 for all-cause mortality (Perlstein 2009). A meta-analysis of 11,827 older adults found a hazard ratio of 1.14 per 1 percentage-point increase (Patel 2010). It is not established whether RDW is a risk factor or a marker of other problems such as inflammation, oxidative stress, poor nutrition or impaired erythropoiesis. Analytical and demographic issues limit its use. Reference limits in the sources used run 11.6–12.0 (lower) and 14.8–15.4 (upper).
LOINC 788-0 (primary), 30385-9. Specimen: whole-blood.
Sources
- CBC, Platelet, No Differential (test 028142), sample report, Labcorp (2020)
- CBC With Differential/Platelet (test 005009), sample report, Labcorp (2024)
- Hematopathology reference ranges, American Board of Pathology
- CBC reference intervals (HEM-2597.01), Te Whatu Ora Health New Zealand, Waikato (2024)
- Red blood cell distribution width and mortality risk in a community-based prospective cohort, Archives of Internal Medicine (2009)
- Red cell distribution width and mortality in older adults: a meta-analysis, The Journals of Gerontology: Series A (2010)
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