What it measures.
Red Cell Distribution Width (RDW) measures the variation in size among your red blood cells. While originally used to classify anemias, RDW has emerged as one of the strongest predictors of all-cause mortality in large population studies—even in people without anemia. Higher RDW reflects biological aging, systemic dysfunction, and poor prognosis across virtually every chronic disease.
The coefficient of variation in red blood cell volume (size), expressed as a percentage. Higher RDW means more size variability among RBCs—a mix of unusually small and large cells rather than uniform cells.
Why it matters.
Elevated RDW predicts death from heart disease, cancer, respiratory disease, and virtually every major cause—independent of other risk factors. It's a marker of cellular 'disorder' reflecting oxidative stress, inflammation, and impaired erythropoiesis. In longevity medicine, RDW deserves more attention than it typically receives.
Physiology.
Healthy bone marrow produces uniform RBCs. Increased RDW occurs when there's variation in RBC production—either from nutritional deficiencies (iron, B12, folate), inflammation, oxidative stress, or bone marrow dysfunction. It also rises when young and old RBCs coexist (reticulocyte release, hemolysis). RDW essentially measures the 'orderliness' of RBC production.
Testing & preparation.
How to prepare
- No special preparation required
- Part of standard CBC
When to test
Included in standard CBC; specifically valuable for longevity assessment and risk stratification
How often
Annually; more often if elevated or investigating anemia
Interpretation.
High red cell distribution width
Common causes:
- Iron deficiency (even without anemia)
- B12 or folate deficiency
- Mixed anemias
- Chronic inflammation
- Oxidative stress
- Chronic diseases (heart failure, kidney disease, cancer)
- Aging
- Blood transfusions (mixing populations)
Implications:
- Strongly predicts all-cause mortality
- Associated with cardiovascular events
- May indicate occult nutritional deficiency
- Reflects systemic biological dysfunction
- Warrants investigation even with normal hemoglobin
Low red cell distribution width
Common causes:
- Efficient, uniform erythropoiesis
- Optimal nutritional status
Implications:
- Associated with lower mortality risk
- Indicates healthy bone marrow function
- Very low values (<11%) are uncommon but not concerning
Optimization.
Diet
- Ensure adequate iron, B12, and folate (common hidden deficiencies)
- Anti-inflammatory diet (Mediterranean pattern)
- Adequate protein for hemoglobin synthesis
Lifestyle
- Regular exercise (reduces inflammation)
- Stress management (cortisol increases RDW)
- Quality sleep (poor sleep increases RDW)
- Avoid smoking (oxidative stress elevates RDW)
Supplements
- Iron if ferritin <50 ng/mL
- B12 if levels suboptimal (<500 pg/mL)
- Folate from diet or supplements
- Omega-3s for anti-inflammatory effect
FAQs.
Why does RDW predict mortality so powerfully?
RDW is a 'systems' biomarker—it reflects the combined effects of inflammation, oxidative stress, nutritional deficiencies, and aging on bone marrow function. It captures biological dysfunction that single markers miss. In large studies, RDW outperformed traditional risk factors in predicting death from any cause.
My RDW is high but hemoglobin is normal—is that concerning?
Yes, this deserves attention. Elevated RDW without anemia (called 'isolated high RDW') still predicts higher mortality. It often indicates early-stage nutritional deficiency (iron, B12, or folate) before anemia develops, or low-grade inflammation. Check ferritin, B12, folate, and inflammatory markers.
At what RDW level should I be worried?
RDW >14.5% is above the reference range and warrants investigation. RDW >15.5-16% is significantly elevated. However, mortality risk increases continuously—even 'normal' RDW of 14% carries more risk than 12%. The optimal is probably <13%.