Blood glucose and HbA1c: what do they show?

Blood glucose and glycated haemoglobin (HbA1c) describe different aspects of glycaemia. A glucose measurement shows blood sugar at a particular point in time, whereas HbA1c reflects average glucose exposure over roughly the previous two to three months, with greater weight given to the more recent weeks.

Which thresholds are commonly used?

For non-pregnant adults, American Diabetes Association criteria classify HbA1c below 5.7% as below the prediabetes range, 5.7–6.4% as prediabetes, and ≥6.5% as a diagnostic threshold for diabetes. For fasting plasma glucose, 100–125 mg/dL indicates impaired fasting glucose and ≥126 mg/dL is a diagnostic threshold for diabetes.

In the absence of unequivocal hyperglycaemia, a result meeting a diagnostic threshold generally requires confirmation with a second abnormal result.

Why can HbA1c disagree with glucose results?

HbA1c depends on red-blood-cell lifespan and on the measurement method. Anaemia, haemoglobin variants, recent bleeding or transfusion, dialysis, pregnancy and other conditions may affect its reliability. A persistent discrepancy between HbA1c and measured glucose deserves medical assessment.

What does “prediabetes” mean?

Prediabetes does not mean that a person already has type 2 diabetes. It does indicate increased future risk and creates an opportunity for early intervention. Regular physical activity, improving diet quality and, where appropriate, weight reduction can substantially lower the risk of progression.

Scientific sources

1. American Diabetes Association — Standards of Care in Diabetes 2026: Diagnosis and Classification.
2. American Diabetes Association — Diabetes Diagnosis & Tests.
3. Johns Hopkins Medicine — Type 2 Diabetes.
4. CDC — A1C Test for Diabetes and Prediabetes.

Last scientific review: 25 August 2026.