RDW optimal range and reference range

Compiled by Alina Radu, founder of Sera, from the published sources below · Last reviewed November 9, 2026

Sera's bands for RDW:

  • Optimal 11.5 – 12.5 %; sufficient 11 – 14.5 %; out of range below 11 or above 14.5 %.

Evidence grade: Published reference interval. A band that primarily reflects a published adult laboratory or population reference interval. It is not necessarily applicable to every assay or laboratory.

All units

UnitOptimalSufficientOut of range
%11.5 – 12.511 – 14.5below 11 or above 14.5

What the evidence says

The acceptable band of 11.0 to 14.5% is the adult interval of the University of Washington laboratory; an NHS laboratory uses 11.5 to 15.7%, so the upper limit depends on the analyser. MedlinePlus says a low RDW is not a sign of anaemia. The optimal band of 11.5 to 12.5% is a functional target: cohort data link a higher RDW to higher mortality without fixing a percentage.

Sources

  • RDW CV | Laboratory Test Guide — Department of Laboratory Medicine & Pathology, UW Medicine (University of Washington), 2022 · Laboratory / reference-interval sourceReference range for RDW-CV from age 6 years, both sexes: 11.0 to 14.5% (effective September 2022). Sera's sufficient band exactly.
  • Full Blood Count (Haematology) — Liverpool Clinical Laboratories, Royal Liverpool University Hospital (NHS), 2013 · Laboratory / reference-interval sourceRed cell distribution width 11.5 to 15.7% for both sexes. A wider interval than Sera's, showing the upper limit varies by analyser. The page is undated; it is the laboratory's current page and says these ranges replaced the ones used before 2 April 2013.
  • RDW (Red Cell Distribution Width) — U.S. National Library of Medicine (MedlinePlus Medical Tests), 2024 · Government / public-health guidanceA low RDW means the red cells are similar in size; it is not a sign of anaemia and is not usually a concern. The page gives no numbers and says reference ranges vary by laboratory. Supports not flagging a low result.
  • The relationship between red cell distribution width and all-cause and cause-specific mortality in a general population — Pan J, Borné Y, Engström G — Scientific Reports, 2019 · Peer-reviewed publicationIn 27,063 Swedish adults followed for about 20 years, the highest quarter of RDW had 34% higher all-cause mortality than the lowest. RDW was measured as RDW-SD in fL, so the study supports 'lower is better' but not Sera's 12.5% edge.

What RDW (Red Cell Distribution Width) measures

A measure of how much your red blood cells vary in size. A higher number means they are more uneven.

Raised RDW is an independent warning sign for heart problems and overall mortality, and can reveal early nutrient shortfalls of iron, B12, or folate before other blood markers change. An RDW creeping up even while still in range is an early warning of a developing nutrient deficiency.

Related blood cell health markers

See RDW in the full list of biomarker ranges

About the author

Alina Radu is the founder of Sera and a software engineer. The idea started with her own Hashimoto's tests, scattered across years without the big picture. She is not a clinician: these ranges are compiled from the published sources linked on each page, with every source named. Alina Radu on LinkedIn

This page is a reference for practitioners, not medical advice. Sera's bands are practitioner targets for structured review, not diagnostic thresholds, and Sera does not diagnose or recommend treatment. Read a result alongside the interval printed by the issuing laboratory and the client's history and symptoms. Range set: Sera Production, version 1.0, last reviewed November 9, 2026.