Biomarker
HbA1c: your three-month average blood sugar
HbA1c measures the proportion of your haemoglobin that has sugar attached. Because red cells live around three months, it gives an average of your blood glucose over that window — no fasting, no single-day flukes.
What it measures
The result is reported as a percentage (NGSP) or in mmol/mol (IFCC). It is the standard test for diagnosing and monitoring type 2 diabetes, and it usually drifts upward for years before a fasting glucose test turns abnormal.
Reported in: % (NGSP) or mmol/mol (IFCC)
Reference ranges
Diagnostic thresholds from the American Diabetes Association.
Normal
Below 5.7% (below 39 mmol/mol)
Roughly an average glucose under 117 mg/dL.
Prediabetes
5.7–6.4% (39–47 mmol/mol)
Increased risk. Lifestyle intervention at this stage prevents or delays progression in a large share of people.
Diabetes
6.5% and above (48+ mmol/mol)
Diagnostic when confirmed on a second test or with clear symptoms.
Common treatment target
Below 7% for many adults
Targets are individualised — tighter in younger people, looser with frailty.
Laboratory reference ranges vary between analysers and populations. Always read the range printed on your own report.
What moves the number
- Total carbohydrate quality and quantity across the whole three-month window
- Body weight and waist circumference
- Physical activity, especially post-meal walking and resistance training
- Sleep duration and quality
- Anything that changes red-cell lifespan: anaemia, haemoglobin variants, recent transfusion, pregnancy, chronic kidney disease
How Vitals uses this measurement
Vitals flags HbA1c at or above 6.5% as diabetes range and 5.7–6.4% as prediabetes, charts the trend across repeat tests, and pairs it with fasting glucose and insulin to compute HOMA-IR — which often turns abnormal before HbA1c does.
Frequently asked questions
How do I convert HbA1c to average glucose?
Estimated average glucose in mg/dL is 28.7 × HbA1c% − 46.7. An HbA1c of 6.5% works out to roughly 140 mg/dL.
How often should it be retested?
Every three months while you are actively changing something, and every six to twelve months once stable. Testing more often than three months rarely shows a real change.
When is HbA1c unreliable?
With sickle cell trait and other haemoglobin variants, iron-deficiency or haemolytic anaemia, recent blood loss or transfusion, advanced kidney disease, and in pregnancy. A fasting glucose or oral glucose tolerance test is used instead.
Related biomarkers
Sources
- American Diabetes Association, Standards of Care in Diabetes — Classification and Diagnosis
- Nathan DM et al. Translating the A1C assay into estimated average glucose values, Diabetes Care 2008
Educational information only, not medical advice. Discuss your own results with a qualified healthcare professional.
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