HbA1c: Your 3-Month Blood Sugar Average Explained

Glycated haemoglobin (HbA1c) has transformed how we diagnose and manage diabetes. Unlike a fasting glucose, which shows your blood sugar at a single moment, HbA1c reflects your average blood glucose level over the past 2–3 months. This makes it the gold standard for monitoring long-term glucose control.

How HbA1c Works

Glucose in the blood naturally attaches to haemoglobin in red blood cells — a process called glycation. The higher your blood sugar, the more glucose attaches. Since red blood cells live for about 90–120 days, HbA1c gives a reliable average over that period. The result is reported as a percentage of total haemoglobin that is glycated.

Interpreting Your HbA1c

The American Diabetes Association (ADA) and WHO use the following cut-points:

• < 5.7% (< 39 mmol/mol): Normal — not diabetes
• 5.7–6.4% (39–47 mmol/mol): Pre-diabetes — increased risk
• ≥ 6.5% (≥ 48 mmol/mol): Diabetes — on two separate tests
• < 7.0%: Recommended target for most adults with diabetes
• < 8.0%: Acceptable target for elderly or those with hypoglycaemia risk

HbA1c and Average Blood Glucose

The Estimated Average Glucose (eAG) formula converts HbA1c to an average blood glucose value that patients find more intuitive. Roughly: HbA1c 6% ≈ 126 mg/dL, 7% ≈ 154 mg/dL, 8% ≈ 183 mg/dL, 9% ≈ 212 mg/dL, 10% ≈ 240 mg/dL.

Limitations of HbA1c

HbA1c can be falsely low in conditions where red cell lifespan is shortened (haemolytic anaemia, recent blood transfusion) and falsely high in iron deficiency anaemia. In these situations, fasting glucose or continuous glucose monitoring may be more reliable.

• Haemoglobin variants (HbS, HbC, HbE) can interfere with some HbA1c assays
• Results may vary slightly between laboratories
• Always interpret alongside fasting glucose and symptoms