RDW (red cell distribution width) is the number on a complete blood count that says how much your red blood cells vary in size. A low RDW means the cells are close to one uniform volume. A high RDW means the population is mixed, small and large cells side by side, which laboratories call anisocytosis. On its own it names no disease. Read next to hemoglobin and mean corpuscular volume it narrows the cause of an anemia fast, and in large population cohorts it has turned out to be one of the more stubborn predictors of mortality on a routine blood panel.
What it measures
A hematology analyzer sizes every red cell as it passes a sensor and builds a volume distribution. Mean corpuscular volume is the average of that curve; RDW is its width. RDW-CV, the form most reports print, is the standard deviation of red cell volume divided by the mean and given as a percentage, so it moves with cell size as well as with spread. RDW-SD is the raw width of the curve in femtoliters, independent of the average. Both come from the ordinary venous draw. For a deeper reference, see the RDW — red cell distribution width entry at Smart Longevity.
Typical values
| Reading | What it generally means |
|---|---|
| RDW-CV around 11.5-14.5% in adults | Commonly quoted reference interval; exact limits differ by analyzer and laboratory |
| RDW-SD around 39-46 fL | Reported less often; unlike RDW-CV it does not shift just because cells are small |
| Above your laboratory’s upper limit | Anisocytosis: red cell populations of different size are circulating. A flag to explain, not a diagnosis |
| Below the lower limit | Carries no accepted clinical meaning |
Treat those figures as orientation only. Reference intervals are analyzer-specific and differ in children, so the interval printed on your own report is the one that counts. A value drifting upward over several years says more than one reading a decimal above the cut-off.
Why it matters for longevity
A raised RDW is associated in observational cohorts with higher all-cause mortality, cardiovascular events, heart failure outcomes and cancer mortality. The association survives adjustment for anemia, kidney function and inflammatory markers, and it behaves as a gradient rather than a threshold: risk starts climbing while the value is still inside the reference range. Average RDW also rises slowly with age. That is why it appears in blood-based aging algorithms such as PhenoAge, which uses it as one of nine routine blood markers.
Nobody thinks a wide size distribution is itself harmful. The plausible reading is that RDW is a cheap aggregate signal: red cell production depends on iron and B vitamin supply, on inflammation, on oxidative stress and on kidney and liver function, so when several of those systems are strained the marrow releases a less uniform crop of cells. The evidence is observational, and no trial shows that pushing the number down changes an outcome.
What changes it
Nutrient deficiency is the common cause. In developing iron deficiency the RDW often rises before MCV — mean corpuscular volume falls and before hemoglobin drops, which makes it an early hint – and a reason to check Ferritin rather than to act on the RDW alone. Vitamin B12 and folate deficiency widen it from the other direction, by adding oversized cells. Recent bleeding, hemolysis, a transfusion or the burst of young cells that follows starting iron or B12 all raise it temporarily; a rising RDW during treatment usually means the treatment is working. Chronic inflammation, kidney disease, liver disease, heavy alcohol use and untreated thyroid disease keep it up for longer.
Some causes are not levers. Thalassemia trait typically gives small cells with a normal RDW, a classic way to tell it apart from iron deficiency, so there the number is doing diagnostic work rather than pointing at anything you can change. Nothing in diet or training targets RDW directly. If yours is high, the useful move is to find and treat what sits behind it.
Related reading
- Which Blood Tests Reflect Aging and How to Read Them
- What Biological Age Is and How to Measure It
- Chronic Inflammation and Aging: Inflammaging
- PhenoAge
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.