HbA1c

What it is

HbA1c (glycated haemoglobin, or glycohaemoglobin) represents the fraction of haemoglobin in red blood cells that has irreversibly bonded with blood glucose. Since red blood cells live approximately 90 to 120 days, HbA1c reflects the average blood glucose level over the past 2 to 3 months — a “glucose memory” independent of what was eaten before the blood draw.

This makes it a very practical test: it requires no fasting and can be drawn at any time of day. It is used both for diabetes screening and for long-term monitoring of glycaemic control in people with diabetes.

Why it matters in PCOS

In the PCOS context, HbA1c is recommended as part of annual metabolic monitoring in people with cardiometabolic risk (overweight, obesity, family history of diabetes, prior prediabetes). It complements fasting glucose and the OGTT for tracking glycaemic status over time.

It is important to understand its limitations in PCOS: HbA1c is less sensitive than the OGTT for detecting insulin resistance or early impaired glucose tolerance. A normal HbA1c (even at 5.3–5.6%) does not guarantee the absence of significant insulin resistance. In PCOS, the OGTT remains the reference test for initial metabolic evaluation.

HbA1c can also be falsely altered in certain situations: it is underestimated in haemolytic anaemia, recent haemorrhage, or G6PD deficiency (accelerated red cell turnover). It is overestimated in iron deficiency anaemia or thalassaemia. These situations are uncommon but important to recognise when interpreting results.

Normal values and diagnostic thresholds

• Normal: < 5.7% (39 mmol/mol)
• Prediabetes: 5.7–6.4% (39–47 mmol/mol)
• Diabetes: ≥ 6.5% (48 mmol/mol) confirmed
• Therapeutic target for type 2 diabetes (general): < 7% (53 mmol/mol)

When is it measured?

HbA1c is ideally measured annually as part of the metabolic follow-up of people with PCOS who have cardiometabolic risk factors (ESHRE 2023). No fasting is required. It can be requested at the initial workup if fasting glucose is borderline (5.0–5.6 mmol/L) to better characterise risk, and is useful for monitoring people treated with metformin.

Key takeaways

• Reflects average blood glucose over the past 2–3 months
• Normal < 5.7%; prediabetes 5.7–6.4%; diabetes ≥ 6.5%
• No fasting required — practical for monitoring
• Less sensitive than OGTT for early insulin resistance in PCOS
• Can be falsified by haemolytic anaemia, iron deficiency, thalassaemia