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Full Blood Count

Red Cell Distribution Width (RDW)

Red cell distribution width measures how much variation there is in the size of your red blood cells — calculated as the coefficient of variation of the red cell volume curve. Where MCV gives the average size, RDW captures the spread. It is one of the most heavily weighted markers in the Levine PhenoAge composite — small absolute changes have large impacts on the bio-age estimate.

Optimal Ranges

Clinical (NHS) Range

11.5-14.5 %

%

Performance-Optimised Range

<13 %

%

The clinical range defines what is considered medically “normal” — broad enough to cover 95% of the population. The performance range reflects where research and clinical experience suggest most people feel and function at their best.

Why It Matters

Why RDW matters for performance

RDW is one of the most underappreciated longevity markers in routine bloodwork. It rises in iron deficiency before haemoglobin falls, rises in B12 deficiency before MCV climbs, and rises non-specifically with chronic inflammation, oxidative stress, and bone marrow stress. Beyond anaemia, an RDW above 14.5% is independently associated with higher all-cause mortality across virtually every age and disease group studied. The longevity-friendly target is below 13%.

Symptoms

Signs your levels may be off

Low / Deficiency

  • RDW is not symptomatic by itself
  • Low RDW is unusual and usually not clinically meaningful

High / Excess

  • Often asymptomatic until the underlying anaemia advances
  • Fatigue, breathlessness on exertion (when anaemia develops)
  • Reduced exercise tolerance
  • Cold hands and feet (iron pattern)

Dietary Sources

Foods that support RDW levels

RDW falls when underlying micronutrient status improvesIron-rich foods: red meat, liver, mussels, lentilsB12 and folate from the same sources as for MCVAntioxidant-rich foods: berries, dark leafy greens, extra virgin olive oilOmega-3 rich fish for inflammation control

Supplementation

Evidence-based supplementation

RDW is a downstream signal — it doesn't respond to direct supplementation, only to fixing what's driving the variation. The work-up follows ferritin, B12, folate, and hs-CRP. Once the underlying cause is corrected, RDW normalises over 8-12 weeks as the older heterogeneous red cells are replaced. Retest at 12 weeks after any intervention.

Research

Key study

An epigenetic biomarker of aging for lifespan and healthspan

Levine ME, Lu AT, Quach A, et al.

Aging (Albany NY) (2018)

DOI: 10.18632/aging.101414

Related Biomarkers

Inflammation

Ferritin (Iron Stores)

Optimal: 80-150 ng/mL

Vitamins & Minerals

Vitamin B12 (Cobalamin)

Optimal: 500-800 pg/mL

Inflammation

hs-CRP (High-Sensitivity C-Reactive Protein)

Optimal: Below 1.0 mg/L

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This content is for educational purposes only and does not constitute medical advice. Your data suggests areas for optimisation, but any concerns should be discussed with a qualified healthcare professional. If your results flag values outside safe ranges, we recommend consulting your GP.