Lab result explainedBlood count (CBC)Last updated November 1, 2026

RDW blood test: what high and low RDW mean

Red cell distribution width (RDW) · Complete blood count (CBC) · reported in % (RDW-CV) or fL (RDW-SD)

RDW (red cell distribution width) measures how much your red blood cells vary in size: a low number means they are all roughly the same size, a high number means there is a wide mix of small and large cells. Most labs call 11.5 to 14.5 percent normal for RDW-CV, and 39 to 46 fL for RDW-SD. The two most common reasons it is high are iron deficiency, where new small cells mix with older normal ones, and vitamin B12 or folate deficiency, where new large cells do the same.

RDW is a supporting index rather than a diagnosis. Its main job is to help sort out why the MCV is abnormal: a high RDW with a low MCV suggests iron deficiency, a normal RDW with a low MCV suggests thalassemia trait, and a high RDW with a normal MCV can reveal a mixed deficiency that the MCV alone would hide. A high RDW is also one of the earliest laboratory signs of iron deficiency, often rising before the hemoglobin or MCV leaves the normal range.

In the past fifteen years, large population studies have linked a high RDW to a higher risk of heart disease, heart failure and death from any cause, even in people without anemia. That association is real, but it reflects RDW acting as a non-specific marker of inflammation, poor nutrition and ill health, not as a cause of any of them. This page explains what RDW measures, what high and low values mean, the causes in order of likelihood, and what usually happens next.

RDW at a glance

  • RDW measures the variation in red blood cell size (anisocytosis), reported as RDW-CV (a percentage) or RDW-SD (in femtoliters).
  • Normal range: about 11.5 to 14.5 percent for RDW-CV and 39 to 46 fL for RDW-SD; labs differ slightly.
  • High RDW most often means iron, B12 or folate deficiency, recent blood loss or a transfusion; it also rises with chronic illness and inflammation.
  • Low RDW has no known clinical significance; it simply means the cells are uniform in size.
  • RDW is most useful read together with MCV: high RDW plus low MCV points to iron deficiency; normal RDW plus low MCV points to thalassemia trait.
  • Next step: check hemoglobin and MCV on the same report, then ferritin, B12 and folate.

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What RDW measures

As red cells pass through the analyzer one at a time, their volumes are plotted as a distribution curve. RDW describes the width of that curve. RDW-CV is the standard deviation of cell volume divided by the MCV, expressed as a percentage; it is the figure most US labs report. RDW-SD is the actual width of the curve in femtoliters, measured at 20 percent of its peak height, and is independent of the MCV.

The difference matters at the extremes. Because RDW-CV is divided by MCV, a person with very small cells can have a high RDW-CV even when the cells are fairly uniform, and a person with very large cells can have a normal RDW-CV despite real variation. RDW-SD avoids that distortion, which is why some labs report both. The two are usually read in the same direction and the interpretation on this page applies to either.

Red cells live about 120 days, so at any moment your blood contains cells made over the past four months. RDW rises whenever the marrow starts making cells of a different size from the ones already circulating: smaller cells in iron deficiency, larger cells in B12 or folate deficiency or reticulocytosis, or a mixture after a transfusion. Once the whole population has turned over to the new size, RDW falls again, which is why long-standing thalassemia trait has a normal RDW despite small cells.

Normal range for RDW

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

GroupTypical range (% (RDW-CV) or fL (RDW-SD))Notes
Adults, RDW-CV11.5–14.5%Some labs use 11.5–15.5% or 12.0–15.0%.
Adults, RDW-SD39–46 fLSome labs use 37–54 fL; RDW-SD is independent of MCV.
Children11.5–15.0%Slightly wider in infancy.
Newbornsup to 17–18%A mixed population of fetal and adult-type cells widens the range in the first weeks.
Pregnancy11.5–15.0%Mild rises are common as iron demand increases in the second and third trimesters.

High RDW

A high RDW (above about 14.5 percent) means your red cells vary more in size than usual, a state hematologists call anisocytosis. It is a signal that the marrow has recently changed the size of the cells it makes, that two populations of cells are present, or that cells are being damaged or fragmented in the circulation.

Read with MCV it becomes informative. High RDW with low MCV is the classic pattern of iron deficiency. High RDW with high MCV points to B12 or folate deficiency, or to a marrow releasing large young cells after bleeding or hemolysis. High RDW with normal MCV can mean early iron deficiency, a mixed iron and B12 deficiency that averages out, early anemia of chronic disease, or recovery from any of these. A high RDW with everything else normal, in someone who feels well, is often a reflection of chronic low-grade inflammation or borderline nutrition and is a prompt to check ferritin and B12 rather than a cause for alarm.

Common causes

  1. Iron deficiency

    As iron stores fall, the marrow begins producing smaller cells that mix with the older normal-sized population, widening the distribution. RDW typically rises before MCV or hemoglobin drop out of range, making it one of the earliest signs. Ferritin confirms it.

  2. Vitamin B12 or folate deficiency

    Impaired DNA synthesis produces oversized cells that mix with existing normal ones, and fragile cells that break apart, so both large and small fragments appear. RDW is often strikingly high (above 16 to 18 percent) in megaloblastic anemia.

  3. Mixed deficiency

    Iron deficiency plus B12 or folate deficiency, common in celiac disease, after bariatric surgery, in heavy drinkers and in pregnancy, produces small and large cells at once. The MCV can be normal while the RDW is very high; this is one of the few situations where RDW reveals what MCV hides.

  4. Recent blood loss or hemolysis

    The marrow responds to lost cells by releasing reticulocytes, which are larger than mature cells. Their arrival widens the size distribution for several weeks. A reticulocyte count shows the response directly.

  5. Early response to iron, B12 or folate treatment

    In the first weeks of treating a deficiency, new normal-sized cells join the old abnormal ones and RDW rises further before it falls. A rising RDW shortly after starting treatment is a sign of response, not failure.

  6. Recent blood transfusion

    Donor cells have their own size and mix with the recipient's, widening the distribution for up to two to three months until the transfused cells age out.

  7. Chronic inflammation, kidney disease, liver disease and heart failure

    Inflammatory cytokines disturb red cell production and shorten cell survival, giving a mildly high RDW with a normal or slightly low MCV. This is the basis of the population studies linking RDW to cardiovascular risk and mortality: RDW is tracking inflammation and ill health in general, not causing it.

  8. Myelodysplastic syndromes

    Abnormal clonal marrow produces cells of inconsistent size. A persistently high RDW with a high MCV, no deficiency and falling platelets or white cells in an older adult should prompt a blood film and hematology review.

  9. Hemoglobinopathies and hereditary spherocytosis

    Sickle cell disease, hemoglobin C disease and spherocytosis produce misshapen cells of variable volume and ongoing hemolysis, keeping RDW high. In hereditary spherocytosis a high RDW with a high MCHC is a characteristic pair.

  10. Red cell fragmentation and lab artefact

    Mechanical heart valves, burns and microangiopathic hemolysis produce fragments that the analyzer counts as tiny cells. Cold agglutinins and stale samples also widen the apparent distribution. A blood film and a fresh sample sort this out.

Symptoms that can go with it

  • Often none; RDW is usually abnormal before any symptoms appear
  • Fatigue and reduced exercise tolerance when anemia is present
  • Pallor, breathlessness on exertion, palpitations
  • Dizziness or headaches
  • Pins and needles, numbness or unsteadiness if B12 deficiency is the cause
  • Restless legs, brittle nails or cravings for ice if iron deficiency is the cause
  • Jaundice or dark urine if hemolysis is the cause

Low RDW

A low RDW (below about 11.5 percent) means your red cells are unusually uniform in size. It has no known disease significance. There is no condition that is diagnosed by a low RDW, and a low value needs no follow-up on its own.

Where a low or low-normal RDW does matter is in context. In someone with small cells (low MCV), a normal or low RDW supports thalassemia trait over iron deficiency, because thalassemia trait produces consistently small cells from birth while iron deficiency produces a mixed population. In someone with mild anemia and a normal MCV, a normal RDW is a point in favor of anemia of chronic disease rather than a nutritional deficiency.

Common causes

  1. Normal variation

    The lower limit of the reference range is set so that about 2.5 percent of healthy people fall below it. A low RDW in someone with normal hemoglobin and MCV is simply a healthy, uniform red cell population.

  2. Thalassemia trait

    A lifelong, stable production of small cells gives a narrow distribution. The combination of a low MCV, a high red cell count and a normal or low RDW is the classic thalassemia trait pattern and distinguishes it from iron deficiency, where RDW is high.

  3. Anemia of chronic disease

    The mild, stable anemia of long-standing inflammation produces cells that are consistently normal-sized or slightly small, so RDW stays normal or low. A high RDW in this setting suggests a coexisting iron deficiency.

  4. Analyzer calibration and method differences

    RDW values are not fully standardized between instrument manufacturers; the same sample can read a point or so lower on one analyzer than another. A borderline low result that changes between labs reflects the machine, not the person.

Symptoms that can go with it

  • None attributable to the RDW itself
  • Any symptoms present relate to the underlying condition, if there is one (for example mild fatigue in anemia of chronic disease)

Read it alongside

  • MCV — the essential partner; the RDW-MCV combination classifies most anemias (Bessman classification).
  • Hemoglobin — determines whether anemia is present at all; a high RDW with a normal hemoglobin is a prompt to check stores, not a diagnosis.
  • MCH and MCHC — confirm hypochromia in iron deficiency; a high MCHC with a high RDW points to hereditary spherocytosis.
  • Ferritin — the first test after a high RDW with a low or normal MCV.
  • Vitamin B12 and folate — the first tests after a high RDW with a high MCV, and worth checking with a normal MCV if iron is deficient, to catch a mixed picture.
  • Reticulocyte count — high when the marrow is responding to blood loss, hemolysis or treatment; it explains many transient RDW rises.
  • CRP — a raised CRP alongside a high RDW suggests inflammation as the driver and makes ferritin harder to interpret.
  • Peripheral blood smear — shows whether the variation is from small cells, large cells, fragments or abnormal shapes.

What usually happens next

  1. Check hemoglobin and MCV on the same report. A high RDW with both normal usually warrants a ferritin, B12 and folate check but no urgency; a low RDW with both normal needs nothing.
  2. For a high RDW with a low MCV, ask for ferritin and CRP. Low ferritin confirms iron deficiency and the next question is the cause. Normal ferritin with a high red cell count points to thalassemia trait and hemoglobin electrophoresis.
  3. For a high RDW with a high MCV, ask for B12, folate, TSH, liver enzymes and a reticulocyte count, and mention alcohol intake and any recent bleeding.
  4. For a high RDW with a normal MCV, ask for ferritin, B12 and folate together, since a mixed deficiency is likely, plus a reticulocyte count if there has been bleeding or hemolysis.
  5. If RDW stays high with no deficiency, no inflammation and no recent transfusion, especially with a high MCV and low platelets after age 60, ask for a blood film review and a hematology opinion.
  6. Recheck in 8 to 12 weeks after treatment starts. Expect RDW to rise briefly first as new cells arrive, then fall to normal over three to four months.

Diet, lifestyle and natural levers with evidence

What can move RDW once the cause is known. Nothing here replaces finding the cause.

Correcting iron deficiency (food plus iron once ferritin confirms it)

Strong evidence

Because iron deficiency is the leading cause of a high RDW, restoring iron stores is the lever with the best evidence: in treatment trials RDW rises for two to four weeks as new cells appear, then falls to normal within three to four months. Heme iron foods with vitamin C, and 40 to 65 mg of elemental iron on alternate days when ferritin is low, are the standard approach.

Confirm low ferritin before taking iron; a high RDW from B12 deficiency, hemolysis or inflammation will not respond and extra iron is not harmless.

Vitamin B12 and folate (when deficient)

Strong evidence

High-dose oral B12 (1,000 to 2,000 mcg daily) or folic acid (400 to 1,000 mcg daily) corrects the respective deficiency within weeks in randomized trials, and the RDW normalizes as the megaloblastic cells are replaced over two to three months. Food sources are animal products and fortified foods for B12, and greens, legumes and fortified grains for folate.

Check B12 before taking folate alone, because folate can mask B12 deficiency while nerve damage progresses.

Vitamin C with plant-based iron

Moderate evidence

Adding 50 to 100 mg of vitamin C to a meal roughly doubles non-heme iron absorption in isotope studies. For someone with a mildly high RDW and low-normal ferritin from a largely plant-based diet, this plus timing tea and coffee away from meals is often enough to restore iron status over several months.

Regular physical activity

Preliminary evidence

In large cross-sectional studies, including US NHANES data, people meeting activity guidelines had lower RDW and the mortality association of a high RDW was weaker in active people. No trial has shown exercise lowers RDW directly; the effect probably runs through lower inflammation and better overall health.

Mediterranean-style diet and stopping smoking

Preliminary evidence

Observational studies link smoking and diets high in processed food to higher RDW, and Mediterranean-style eating to lower RDW, consistent with RDW tracking inflammation. These are sensible changes with broad benefit, but there is no trial evidence that they lower RDW specifically, so expect modest, slow shifts at most.

Omega-3 fatty acids

Preliminary evidence

A few small studies report lower RDW with higher omega-3 intake or red cell omega-3 levels, consistent with reduced inflammation and more stable red cell membranes. The evidence is weak and indirect; omega-3 should not be taken to fix an RDW, and a high RDW still needs its cause found.

When to see a clinician promptly

  • A hemoglobin under 10 g/dL on the same report, or any anemia with chest pain, breathlessness at rest, fainting or a racing heartbeat.
  • A high RDW with low MCV and black, tarry or bloody stools, vomiting blood or unexplained weight loss, which point to gastrointestinal bleeding.
  • A high RDW with a high MCV and numbness, tingling, poor balance or confusion, which can mean B12 deficiency is affecting the nerves.
  • A high RDW with jaundice, dark urine or fever, suggesting active hemolysis.
  • A persistently high RDW with low platelets or white cells, particularly after age 60, which needs a blood film and specialist review.

Frequently asked questions

What is a normal RDW level?

About 11.5 to 14.5 percent for RDW-CV, which is what most US reports show, and about 39 to 46 fL for RDW-SD. Some labs set the upper limit at 15 or 15.5 percent. Newborns run higher and pregnancy nudges it up. Compare your result with the range on your own report.

Is an RDW of 15 or 16 percent bad?

It is mildly high and rarely dangerous on its own. It most often means iron stores are running low, a B12 or folate level is borderline, there has been recent bleeding, or there is some chronic inflammation. If hemoglobin and MCV are normal, there is no urgency, but a ferritin, B12 and folate check is worthwhile because RDW is often the first index to move.

What does a high RDW mean if everything else is normal?

Usually early iron deficiency, a mixed mild deficiency, recovery from a recent illness or bleed, or chronic low-grade inflammation. It is not a diagnosis. Ferritin, B12 and folate find a cause in most cases; if they are normal and you feel well, a repeat CBC in two to three months is reasonable.

What is the difference between RDW-CV and RDW-SD?

RDW-CV divides the spread of cell sizes by the average size and reports a percentage; RDW-SD reports the spread itself in femtoliters without reference to the average. RDW-CV can read high simply because the cells are small (low MCV), even if they are fairly uniform; RDW-SD is not affected by that. When both are reported and disagree, RDW-SD is the better measure of true variation.

Is a high RDW a sign of cancer?

Not in itself. A high RDW is overwhelmingly due to nutritional deficiency, blood loss or inflammation. Some cancers raise RDW indirectly through chronic inflammation or through bleeding that causes iron deficiency, and a few blood cancers (myelodysplasia, some leukemias) raise it directly, but in those cases other parts of the CBC are usually abnormal too. A high RDW alone does not warrant a cancer work-up.

Does a high RDW mean heart disease?

Large studies have found that people with a higher RDW are more likely to develop heart disease and heart failure and to die earlier, even without anemia. The best explanation is that RDW is a general marker of inflammation, poor nutrition and chronic illness, which drive both the RDW and the heart risk. Lowering RDW itself has never been shown to change outcomes; addressing the underlying causes is what matters.

Can RDW be low? What does it mean?

Yes, and it means your red cells are uniform in size. No disease is diagnosed by a low RDW and it needs no follow-up. Its only use is as a supporting clue: with small cells, a normal or low RDW points to thalassemia trait rather than iron deficiency.

Why did my RDW go up after I started iron?

That is expected and is a sign the treatment is working. The new, normal-sized cells your marrow is making join the small iron-deficient cells already in circulation, so for a few weeks there are two populations and RDW rises. As the old cells die off over three to four months, RDW falls back to normal.

References

  1. RBC indices. MedlinePlus Medical Encyclopedia, National Library of Medicine (2024)View source
  2. Complete blood count (CBC). Mayo Clinic (2023)View source
  3. Bessman JD, Gilmer PR, Gardner FH. Improved classification of anemias by MCV and RDW. American Journal of Clinical Pathology (1983)
  4. Salvagno GL, Sanchis-Gomar F, Picanza A, Lippi G. Red blood cell distribution width: A simple parameter with multiple clinical applications. Critical Reviews in Clinical Laboratory Sciences (2015)
  5. Perlstein TS, Weuve J, Pfeffer MA, Beckman JA. Red blood cell distribution width and mortality risk in a community-based prospective cohort. Archives of Internal Medicine (2009)
  6. Patel KV, Semba RD, Ferrucci L, et al.. Red cell distribution width and mortality in older adults: a meta-analysis. Journals of Gerontology Series A (2010)
  7. Lippi G, Plebani M. Red blood cell distribution width (RDW) and human pathology. One size fits all. Clinical Chemistry and Laboratory Medicine (2014)
  8. Van Vranken M. Evaluation of Microcytosis. American Family Physician (2010)View source
  9. Evaluation of Anemia. Merck Manual Professional Version (2024)View source
  10. Iron: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements (2024)View source
  11. Vitamin B12: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements (2024)View source
  12. Anemia. American Society of Hematology (2024)View source

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.