Red cell distribution width

A measure of how variable red blood cell size is, originally used to classify anemia, now recognized as a broad predictor of mortality risk.

What it measures

Every complete blood count reports RDW, the coefficient of variation of red cell volume. A low value means the cells are uniform in size. A high value means the population is mixed, usually because the bone marrow is producing cells under changing conditions.

Its original use was practical: in iron deficiency, RDW rises early, which helps separate iron deficiency from thalassemia trait when both show small cells.

Why it turned out to matter more

Since 2009, large cohort analyses have repeatedly found that elevated RDW predicts all-cause and cardiovascular mortality in people without anemia, independently of hemoglobin, iron status, inflammation and kidney function. It carries the largest weight of any blood marker in the PhenoAge biological age algorithm.

Nobody has a clean single explanation. The leading interpretation is that RDW captures systemic stress on erythropoiesis, from inflammation, oxidative stress, nutrient handling and marrow environment, that no single test names directly.

What to do with a high RDW

First look for a cause: iron, B12 or folate deficiency, recent blood loss, hemolysis, liver disease, or a mixed picture after supplementation started. A treatable cause is the common answer.

If nothing is found, an isolated modestly elevated RDW is not a diagnosis and there is no treatment aimed at the number. Treat what raised it, and read it as one input among several rather than a verdict.

Reference ranges

  • Typical laboratory reference: roughly 11.5 to 14.5 percent. Analyzer dependent. Population reference range, not a personal target.

RDW comes with any standard blood count, and it is one of the nine inputs in our phenotypic age calculator.

References

  1. Patel KV, et al. Red cell distribution width and mortality in older adults: a meta-analysis. J Gerontol A Biol Sci Med Sci. 2010.
  2. Perlstein TS, et al. Red blood cell distribution width and mortality risk in a community-based prospective cohort. Arch Intern Med. 2009.
  3. Levine ME, et al. An epigenetic biomarker of aging for lifespan and healthspan. Aging (Albany NY). 2018.