RDW — red cell distribution width — is the single most underrated number on a complete blood count. It measures how much variation there is in the size of your red blood cells. A low number means your cells are uniform; a high number means there are noticeably big and noticeably small cells in the same sample.
Three things make RDW unusually useful. First, it is one of the earliest signals of evolving iron, B12, or folate deficiency, often rising before MCV becomes abnormal. Second, it is a quietly powerful predictor of all-cause and cardiovascular mortality in large cohort studies — independent of hemoglobin and other CBC values. Third, it costs nothing extra: it is calculated automatically on every CBC.
What RDW measures
RDW is the coefficient of variation of red cell volumes — essentially the standard deviation of cell sizes divided by the mean, expressed as a percentage. A modern automated analyzer measures thousands of individual cells and reports a single number that captures how spread out the size distribution is.
Conceptually: if every red cell is exactly 90 fL, RDW is low. If half the cells are 75 fL and half are 105 fL, the mean MCV might still be 90 — but RDW would be high. That is why RDW catches what MCV alone cannot: two populations of cells, which usually means two processes happening at once or a deficiency that is partially treated.
RDW reference range
| Grupo demográfico | Bajo | Alto | Unidad |
|---|---|---|---|
| Normal | 11.5 | 14.5 | % |
| Mildly elevated | 14.6 | 16 | % |
| Significantly elevated | 16.1 | 30 | % |
- Normal: 11.5–14.5%.
- Mildly elevated: 14.6–16.0% — often early nutritional deficiency or recent treatment of one.
- Significantly elevated: above 16.0% — strong signal of an active anemia process, multiple coexisting deficiencies, or hematologic disease.
Different analyzers may report RDW-CV (the percentage above) and RDW-SD (in fL). RDW-CV is the more commonly reported value.
What high RDW means
The rough framework: high RDW means cells of multiple sizes are circulating. Common causes:
- Iron deficiency anemia — RDW is one of the earliest values to rise, often before MCV drops. Useful early warning.
- B12 or folate deficiency — high RDW with high MCV is the classic megaloblastic pattern.
- Mixed deficiency — coexisting iron and B12/folate deficiency. MCV may be near-normal because the small iron-deficient cells and large B12-deficient cells average out, but RDW is markedly elevated.
- Recent transfusion — donor cells of slightly different size mixed with native cells.
- Hemolysis — reticulocytosis releases large young cells alongside the smaller older cells.
- Recent recovery from anemia — new normal-size cells alongside old smaller cells.
- Liver disease, chronic kidney disease, myelodysplastic syndromes.
- Hemoglobinopathies — sickle cell, thalassemia (variable; thalassemia trait often has surprisingly normal RDW).
The diagnostic shortcut: in microcytic anemia, high RDW favors iron deficiency, while normal RDW favors thalassemia trait. This is one of the cleanest discriminators in routine practice.
Low RDW
Low RDW (uniform cell size) is favorable. There is no clinically meaningful "too low" RDW — values at the low end of the reference range simply mean the red cell population is healthy and uniform. No action needed.
Why RDW gets ignored — and why it shouldn't
Patient portals do not flag mildly high RDW the way they flag a slightly low hemoglobin. Yet RDW carries real prognostic weight:
- Cardiovascular outcomes — elevated RDW independently predicts heart failure progression, mortality post-myocardial infarction, and stroke outcomes in multiple large cohort studies.
- All-cause mortality — adults with RDW above 14.5% have meaningfully higher 5- and 10-year mortality rates than those with RDW under 13%, even after adjusting for hemoglobin, age, sex, and comorbidities.
- Septic outcomes — RDW on admission predicts ICU mortality in sepsis better than several traditional severity scores in some studies.
The mechanism is unclear. RDW likely tracks chronic inflammation, oxidative stress, and disturbed erythropoiesis — the kind of slow systemic dysfunction that shows up before any single test goes overtly abnormal. It is best treated as a "general illness" signal worth investigating.
The trend matters: a stable RDW of 14.0% is benign. An RDW that has crept from 12.5% to 15.5% over a year is a flag — usually for evolving deficiency or chronic inflammation.
Rastrea este biomarcador a lo largo del tiempo en AskAnything.health — sube tus resultados de laboratorio y ve las tendencias de un vistazo.
When RDW deserves a workup
- RDW above 16% with anemia — pursue iron, B12, folate, and TSH; consider mixed deficiency.
- RDW above 16% without overt anemia — early deficiency or chronic inflammation; check ferritin, CBC trend, hs-CRP, basic metabolic panel.
- Stable RDW with rising trend — investigate even if still within reference range.
- RDW > 20% — strongly suggests active hematologic process or significant multi-deficiency; do not delay workup.
Esta información es solo con fines educativos y no sustituye el consejo médico profesional. Siempre consulta a tu proveedor de salud sobre tus resultados de laboratorio.
Tests that complete the picture
- MCV, MCH, hemoglobin, RBC count — full red cell line; RDW interpretation depends on these.
- Ferritin, transferrin saturation — iron status.
- Vitamin B12, folate — macrocytic causes.
- TSH — hypothyroidism causes mild macrocytosis with mildly elevated RDW.
- Reticulocyte count — high reticulocytes elevate RDW transiently.
- Hemoglobin electrophoresis — when thalassemia is suspected.
- hs-CRP — chronic inflammation.
- Liver and kidney function — both can disturb erythropoiesis and raise RDW.