HbA1c: the key marker of prediabetes and diabetes

HbA1c reflects your average blood glucose over two to three months. Prediabetes and diabetes thresholds, the marker's limitations, and what you need to know.

HbA1c: the key marker of prediabetes and diabetes

5 min - BioSense Team - 2026-06-24

  • level:understanding
  • hba1c
  • prédiabète
  • diabète
  • hémoglobine glyquée
  • glycémie

Glycated haemoglobin — HbA1c — reflects your average blood glucose over the last two to three months. It is the reference marker for spotting prediabetes or diabetes, and for tracking their progression. This article explains what it really measures, the thresholds to know, its limitations — and why prediabetes is a precious window for action.

You see "HbA1c" on your blood test report. The figure reads 5.8%, 6.1%, perhaps 5.4%. No alarming asterisk, but you're not sure what it really means. You know it has something to do with sugar, but what exactly?

Here's what this marker really says about your metabolism, which thresholds to remember, and why it is never read entirely on its own.

What exactly is HbA1c?

HbA1c — short for glycated haemoglobin — is a particular form of haemoglobin, the one that has bound to the glucose present in your blood. The higher your average blood glucose, the larger the fraction of your haemoglobin that becomes "glycated." This bond is stable: it lasts as long as the red blood cells concerned, about 120 days.

Practical consequence: measuring HbA1c means measuring the average blood glucose integrated over the last two to three months. Where fasting blood glucose gives a one-off snapshot (which can be influenced by your meal the day before, your stress, your physical activity), HbA1c gives a more stable overall view. That is precisely what makes it the reference marker for spotting a chronic disturbance of glucose metabolism.

HbA1c is expressed as a percentage (in France, as in the United States) or in millimoles per mole (mmol/mol, the unit of the International Federation of Clinical Chemistry). Both units coexist. For reference, 6.5% corresponds to 48 mmol/mol.

Key point — HbA1c reflects your average blood glucose over the last two to three months. More stable and more integrative than a one-off glucose reading, it is the reference marker of glucose metabolism.

The thresholds to know

The thresholds used in practice are largely harmonised across French, European and American guidelines.

  • Below 5.7% (39 mmol/mol): values considered normal in a non-pregnant adult with no particular condition.
  • Between 5.7% and 6.4% (39-46 mmol/mol): the prediabetes zone. Glucose intolerance is setting in; the risk of progressing to type 2 diabetes is significantly increased.
  • At or above 6.5% (48 mmol/mol): the diabetes zone. To be confirmed by a second measurement at a later date, or by a fasting glucose, or by an oral glucose tolerance test according to the guidelines in force.

Important: a single HbA1c measurement above 6.5% is not enough to diagnose diabetes. The diagnosis relies on confirmation by a second sample, or by another method (fasting glucose above 1.26 g/L on two occasions, or an oral glucose tolerance test). This precaution avoids errors linked to an isolated measurement.

Key point — Normal below 5.7%, prediabetes between 5.7% and 6.4%, diabetes at 6.5% and above — to be confirmed by a second measurement. Three simple zones to memorise.

Prediabetes: a precious window for action

The prediabetes zone — between 5.7% and 6.4% HbA1c — is a transitional stage, not an inevitability. Without intervention, around 5 to 10% of people with prediabetes progress each year to confirmed type 2 diabetes. But this progression is neither rapid nor inevitable.

Several major international studies, including the American Diabetes Prevention Program (DPP) and the Chinese Da Qing Study, have shown that a lifestyle intervention could reduce by more than 50% the risk of progression to diabetes in people with prediabetes. The documented levers are well known: a balanced diet with a moderate glycaemic index, regular physical activity (at least 150 minutes of moderate-intensity activity per week, according to WHO and HAS guidelines), losing 5 to 7% of initial body weight in cases of excess weight.

Spotting prediabetes in time is therefore not an admission of failure, it is an opportunity. That is precisely why identifying an HbA1c of 5.9% or 6.1%, even when it appears "normal" on some reports, can change the long-term trajectory.

Key point — Prediabetes is not an inevitability. International studies show that a lifestyle intervention reduces by more than 50% the risk of progression to diabetes. Spotting an HbA1c between 5.7% and 6.4% is an opportunity for action, not a verdict.

The limitations of HbA1c you should know

HbA1c is valuable, but it can be skewed — in either direction — by several situations that alter the lifespan of your red blood cells or the structure of your haemoglobin. This is a little-known point, even though it is essential to avoid misinterpreting a result.

Situations that can falsely lower HbA1c

Anything that shortens the life of your red blood cells, or increases their turnover, tends to lower HbA1c without your average blood glucose actually being lower: haemolytic anaemia, certain haemoglobinopathies (sickle cell disease, thalassaemias), pregnancy (faster red-cell turnover), severe chronic kidney failure treated with erythropoietin, recent blood donations.

Situations that can falsely raise HbA1c

Conversely, anything that prolongs the life of red blood cells can artificially raise HbA1c: uncorrected iron deficiency, splenectomy, certain rare bone-marrow diseases.

Why this matters

In practice, a pregnant woman with an apparently low HbA1c should not conclude that all is well on the glucose front — the measurement is skewed by pregnancy. Conversely, a person with significant iron deficiency may have an HbA1c overestimated relative to their real average blood glucose. In these situations, fasting glucose, or even the oral glucose tolerance test, remain the references. Your doctor may also suggest a fructosamine measurement, which reflects blood glucose over two to three weeks independently of red blood cells.

Key point — HbA1c can be skewed by anything that alters the lifespan of your red blood cells: anaemia, haemoglobinopathies, pregnancy, severe kidney failure. In these cases, other markers take over. Reading in context remains essential.

The BioSense method

HbA1c perfectly illustrates why a biomarker, even a well-standardised one, only makes sense when read in context. BioSense systematically cross-references your HbA1c with your health questionnaire (age, sex, personal and family history, lifestyle, current treatments, pregnancy status, history of anaemia or haemoglobinopathies, reason for the prescription), with your CBC and your fasting glucose when available, and with official guidelines. Your results are also tracked over time — particularly useful for spotting a drift from the normal zone toward the prediabetic zone.

*ChatGPT answers you. BioSense knows you.* *BioSense, your prevention companion.* BioSense does not provide a diagnosis and does not replace your doctor. It is a prevention companion that helps you understand, ask the right questions and act day to day.

Preparing for your medical appointment: three useful questions

If your HbA1c raises questions, here are three to prepare for your next consultation:

  • Given my history and my lifestyle, is my HbA1c optimal for me, or merely "within average"?
  • If it is between 5.7% and 6.4% (prediabetes), which lifestyle levers would have the most impact in my case, and how often should I have a follow-up check?
  • Are there situations in my profile (iron deficiency, possible pregnancy, chronic anaemia) that could skew my HbA1c and call for another complementary test?

These questions avoid the trap of "all is well, everything is within range" and steer the consultation toward a genuine reading of your metabolic trajectory.

Import your blood test into BioSense and receive a plain-language reading of your HbA1c and all your biomarkers, contextualised to your personal profile, compared against official guidelines and tracked over time. Your data is hosted in France on infrastructure certified for hosting health data (HDS).

[→ Import my report](/en/onboarding/upload)

Sources and methodology

  • American Diabetes Association (ADA) — Standards of Care in Diabetes — 2026 (Diabetes Care 2026;49(Suppl_1)): HbA1c thresholds for prediabetes (5.7-6.4% / 39-47 mmol/mol) and diabetes (≥ 6.5% / 48 mmol/mol).
  • Haute Autorité de Santé (HAS) — Care pathway for the adult patient living with type 2 diabetes (Guide, published 16 July 2025) and Therapeutic strategy for the patient living with type 2 diabetes (good-practice recommendation, June 2024).
  • Collège National des Gynécologues et Obstétriciens Français (CNGOF) and Société Francophone du Diabète (SFD) — Gestational diabetes: recommendations for clinical practice (J Gynecol Obstet Biol Reprod 2010, 39:S1-S342).
  • European Association for the Study of Diabetes (EASD) and ADA — Consensus Reports on the diagnosis and management of type 2 diabetes.
  • World Health Organization (WHO) — Definition, Diagnosis and Classification of Diabetes Mellitus and its Complications: Report of a WHO Consultation (1999); Use of Glycated Haemoglobin (HbA1c) in the Diagnosis of Diabetes Mellitus (Abbreviated Report of a WHO Consultation, 2011).
  • Knowler WC, Barrett-Connor E, Fowler SE et al. — Reduction in the Incidence of Type 2 Diabetes with Lifestyle Intervention or Metformin (Diabetes Prevention Program Research Group, New England Journal of Medicine 2002;346(6):393-403): 58% reduction with lifestyle intervention, 31% with metformin.
  • Pan XR, Li GW, Hu YH et al. — Effects of diet and exercise in preventing NIDDM in people with impaired glucose tolerance: the Da Qing IGT and Diabetes Study (Diabetes Care 1997;20(4):537-544); Li G, Zhang P, Wang J et al. — The long-term effect of lifestyle interventions to prevent diabetes in the China Da Qing Diabetes Prevention Study: a 20-year follow-up study (Lancet 2008;371:1783-1789).

*BioSense methodology: our content draws on official French guidelines (HAS, ANSM, Santé Publique France, INSERM, Assurance Maladie, Anses), European guidelines (ESC, EASD, NICE) and international ones (WHO, ADA, AHA, NIH), as well as specialised learned societies (SFE, SFBC, GRIO). It is reviewed with an educational approach before publication.*

Going further

*This article is for informational purposes only and is neither medical advice nor a diagnosis. For any questions about your health, consult your doctor.*