HbA1c 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 HbA1c:
- Optimal 4.5 – 5.4 % (26 – 36 mmol/mol); sufficient 4 – 5.6 % (20 – 38 mmol/mol); out of range above 5.6 %.
Evidence grade: Established clinical threshold. A band edge that corresponds to a recognised diagnostic, risk or treatment threshold from a major clinical guideline or public-health body.
All units
| Unit | Optimal | Sufficient | Out of range |
|---|---|---|---|
| % | 4.5 – 5.4 | 4 – 5.6 | above 5.6 |
| mmol/mol | 26 – 36 | 20 – 38 | above 38 |
Bounded on the high side only — a low result is not treated as out of range. No established lower limit — a low HbA1c reflects red cell lifespan (haemolysis, recent transfusion, pregnancy) rather than better glycaemic control.
What the evidence says
The acceptable band stops at 5.6% (38 mmol/mol), immediately below the ADA's prediabetes threshold of 5.7% (39 mmol/mol). NICE in the UK and Diabetes Canada draw the line higher, at 6.0% (42 mmol/mol). The optimal band ending at 5.4% follows pooled European and US cohort data in which 5.0 to under 5.5% was the lowest-risk group; its lower edge of 4.5% has no published source. A low result is not flagged, but it is not a sign of better control: it usually reflects red cell lifespan, and pooled data show higher mortality below 5.0% in people without diabetes.
Sources
- Diagnosis and Classification of Diabetes: Standards of Care in Diabetes—2026 — American Diabetes Association Professional Practice Committee — Diabetes Care, 2026 · Clinical guidelinePrediabetes by HbA1c is 5.7 to 6.4% (39 to 47 mmol/mol). Sera's sufficient band stops at 5.6% (38 mmol/mol), just below it. Current edition in force.
- Diabetes Testing — Centers for Disease Control and Prevention, 2024 · Government / public-health guidanceA1C: normal below 5.7%, prediabetes 5.7 to 6.4%, diabetes 6.5% or above. Matches Sera's sufficient upper edge.
- Type 2 diabetes: prevention in people at high risk (PH38): Recommendations — National Institute for Health and Care Excellence, 2017 · Clinical guidelineIn the UK, high risk of type 2 diabetes starts at an HbA1c of 42 mmol/mol (6.0%), not 39. Results of 39 to 41 mmol/mol are out of range in Sera but below the NICE high-risk line. Published 2012, last updated September 2017 and still in force.
- Definition, Classification and Diagnosis of Diabetes, Prediabetes and Metabolic Syndrome (Chapter 3) — Diabetes Canada Clinical Practice Guidelines Expert Committee (Punthakee Z, Goldenberg R, Katz P), 2018 · Clinical guidelinePrediabetes by A1C is 6.0 to 6.4% in Canada, a higher line than the ADA's 5.7%. 2018 is the current full edition.
- HbA1c levels in non-diabetic older adults - No J-shaped associations with primary cardiovascular events, cardiovascular and all-cause mortality after adjustment for confounders in a meta-analysis of individual participant data from six cohort studies — Schöttker B, Rathmann W, Herder C, et al. — BMC Medicine, 2016 · Peer-reviewed publicationSix European and US cohorts, 28,681 people without diabetes aged 50 and over: 5.0 to under 5.5% was the lowest-risk reference group. After adjustment, risk was clearly higher only at 6.0 to under 6.5%, and results below 5.0% were inconsistent. Partial support for an optimal band ending at 5.4%.
- Glycated haemoglobin A1c as a risk factor of cardiovascular outcomes and all-cause mortality in diabetic and non-diabetic populations: a systematic review and meta-analysis — Cavero-Redondo I, Peleteiro B, Álvarez-Bueno C, Rodriguez-Artalejo F, Martínez-Vizcaíno V — BMJ Open, 2017 · Peer-reviewed publicationMeta-analysis of 46 studies (not limited to Europe and North America): in people without diabetes the lowest mortality was at 5.0 to 6.0%, with higher all-cause mortality above 6.0% (HR 1.74) and below 5.0% (HR 1.19). Does not support an optimal band reaching down to 4.5%.
- IFCC Standardization of HbA1c: IFCC and NGSP — NGSP (National Glycohemoglobin Standardization Program), 2025 · Laboratory / reference-interval sourceThe master equation linking % (NGSP) and mmol/mol (IFCC). It gives 5.6% = 38, 5.4% = 36, 4.5% = 26 and 4.0% = 20 mmol/mol, the same as Sera's rows.
What HbA1c (Glycated Hemoglobin) measures
Your average blood sugar over the past 2 to 3 months.
Shows how well your blood sugar has been controlled over the longer term. Sera's bands are tighter than the diagnostic thresholds on purpose — the ADA marks prediabetes from 5.7% (39 mmol/mol).