RDW · high result
High RDW: what it means, causes and how to read it with the MCV
Short answer
A high RDW (red cell distribution width) — above about 14.5–15% on most lab ranges — means your red cells vary more than usual in size. It is an early sign of a nutritional deficiency — iron, vitamin B12 or folate — often appearing before hemoglobin or MCV change, and it rises after a transfusion, in hemolysis, liver disease, alcohol use and mixed anemias. Read with the MCV it points to a cause: high RDW with a low MCV is iron deficiency; with a high MCV, B12 or folate; with a normal MCV, early deficiency, mixed deficiency or recovery. A high RDW is never a diagnosis on its own, and with a normal hemoglobin, MCV, ferritin, B12 and folate it needs no treatment — though in population studies it tracks with inflammation and cardiovascular risk, which is why it is sometimes reported as a 'risk marker'.
Medically reviewed by Youmna DiStefano, MD · Last reviewed November 2026
When does this result need same-day care?
- A high RDW is not urgent on its own. Seek same-day care for the anemia it may accompany — hemoglobin under about 8 g/dL, chest pain, breathlessness at rest, fainting, or black or bloody stools.
What is the normal range for RDW?
- age 18+: 11.5 – 14.5 %
Often reflects iron, B12, or folate deficiency, or recent blood loss.
Full reference table, citations and testing options: RDW biomarker page.
What are the common causes of high RDW?
- Iron deficiency — the earliest laboratory sign, often before hemoglobin or MCV fall
- Vitamin B12 or folate deficiency
- Mixed deficiency (iron plus B12 or folate), or early recovery from anemia as new cells of a different size enter the blood
- Recent blood transfusion — two populations of cells for weeks
- Hemolysis, sickle cell disease and thalassemia (especially intermedia), and reticulocytosis after bleeding
- Liver disease and alcohol use
- Chronic inflammation, chronic kidney disease, heart failure and diabetes — modest elevations
- Myelodysplastic syndromes and other marrow disorders (uncommon; usually with other counts abnormal)
What symptoms go with high RDW?
- None from the RDW itself
- Fatigue, breathlessness and pallor when there is anemia; restless legs and ice craving with iron deficiency; tingling and memory change with B12 deficiency
What do clinicians usually check next?
- Ferritin, vitamin B12 and folate — the three tests that explain most high RDWs
- Interpret with the MCV and hemoglobin on the same CBC: low MCV → iron; high MCV → B12/folate; normal MCV with a high RDW → early or mixed deficiency
- A reticulocyte count and a peripheral blood smear when hemoglobin is also low or the cause is unclear
- A liver panel and TSH; a review of alcohol intake and recent transfusions
- CRP if inflammation is suspected, and kidney function
- Repeat the CBC in 8–12 weeks after correcting a deficiency — the RDW rises first as new normal cells mix with old ones, then normalizes
- Nothing further when hemoglobin, MCV, ferritin, B12 and folate are all normal and the RDW is only mildly raised
Follow-up tests available at TestWell: TSH Test (Thyroid Stimulating Hormone) ($20.99), TSH with Reflex to Free T4 Test ($29.99), TSH + Free T4 Test ($27.99), Vitamin B12 Test ($19.99), Ferritin Test ($21.99), C-Reactive Protein Test (CRP) ($26.99).
How do you recheck RDW?
Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.
| Test or panel | Type | Price | Fasting | Results |
|---|---|---|---|---|
| Biological Age Panel | Panel · 3 tests | $59.99 | Yes | 1–3 business days |
| General Wellness Panel | Panel · 4 tests | $49.99 | Yes | 1–3 business days |
| Blood Clotting Panel | Panel · 4 tests | $149.99 | No | 3–7 business days |
| CBC with Differential Test | Test · 14 biomarkers | $17.99 | No | 1–3 business days |
Common questions
High RDW but everything else normal — does it matter?
Often it's the first hint of a developing iron, B12 or folate deficiency, so those three tests are worth checking. On its own it doesn't need treatment.
What is a high RDW?
Above the lab's upper limit, usually 14.5–15% (RDW-CV). Values of 15–17% are mild and typically nutritional or inflammatory; above 20% usually means a significant deficiency, hemolysis, recent transfusion or a marrow disorder.
What does a high RDW with a low MCV mean?
Iron deficiency until proven otherwise — small new cells mixing with older normal ones widen the distribution. Thalassemia trait, by contrast, gives a low MCV with a normal RDW, which is one way the two are told apart before ferritin comes back.
What does a high RDW with a normal MCV mean?
Usually an early deficiency (iron, B12 or folate) that has not yet shifted the average cell size, or a mixed deficiency in which small and large cells cancel out in the MCV. Ferritin, B12 and folate settle it.
Is a high RDW linked to heart disease?
In large population studies a higher RDW is associated with cardiovascular events, heart failure and mortality, probably because it reflects inflammation, nutritional status and kidney function. It is a statistical marker, not something to treat; the actionable step is the deficiency or condition behind it.
How is a high RDW lowered?
By correcting the cause — replacing iron, B12 or folate, treating the liver or inflammatory condition, reducing alcohol. The RDW typically rises further for a few weeks after treatment begins, then normalizes over three to four months as the red cell population turns over.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.