What does the Haemoglobin A1c test measure?

Why is Haemoglobin A1c measured?

HbA1c is the cornerstone glycaemic marker for diabetes screening, diagnosis, and treatment monitoring — endorsed by the ADA, EASD, WHO, and every major diabetes guideline as the primary long-term glucose exposure marker. Its 2–3 month integrated reflection of mean glycaemia overcomes the snapshot limitation of fasting glucose, capturing postprandial and nocturnal glucose excursions as well as fasting levels. HbA1c above 6.5% (48 mmol/mol) on two separate occasions — or once alongside a symptomatic hyperglycaemic presentation — meets WHO diagnostic criteria for type 2 diabetes. In established diabetes management, HbA1c guides therapy intensification decisions and is the primary surrogate endpoint in all major diabetes outcome trials — each 1% absolute reduction in HbA1c is associated with approximately 21% reduction in diabetes-related deaths, 14% reduction in myocardial infarction risk, and 37% reduction in microvascular complication risk in the UKPDS cohort. As a cardiovascular risk marker in Boston Heart Diagnostics and functional medicine cardiometabolic panels, HbA1c quantifies the glycaemic contribution to ASCVD risk independent of fasting glucose, with levels in the prediabetes range associated with measurably elevated cardiovascular event rates.

Reference ranges

Standard reference

Normal: <5.7% (<39 mmol/mol); Prediabetes: 5.7–6.4% (39–46 mmol/mol); Diabetes: ≥6.5% (≥48 mmol/mol) # REVIEWER FLAG: These are ADA diagnostic thresholds, not laboratory reference ranges in the traditional sense — confirm how LabCorp formats HbA1c results on their report. Most platforms report a numerical HbA1c percentage alongside ADA-aligned interpretive categories (normal, prediabetes, diabetes). Confirm whether LabCorp reports IFCC units (mmol/mol) alongside NGSP units (%) as dual reporting, and confirm the specific LabCorp assay methodology — ion-exchange HPLC (e.g. Bio-Rad Variant II) and immunoassay are the dominant platforms; methodology determines susceptibility to haemoglobin variant interference.

Functional medicine optimal

<5.4% (<36 mmol/mol) (functional medicine and preventive cardiology optimal — below the prediabetes threshold and within a range associated with minimal glycation-related end-organ and vascular risk; some integrative practitioners target ≤5.2% for maximum metabolic health). # REVIEWER FLAG: The <5.4% functional optimal is used in integrative and preventive medicine practice but is below any formal clinical guideline intervention threshold — reviewer to confirm appropriate framing. Note that excessively low HbA1c (<4.5%) in the absence of haemolysis raises concern for hypoglycaemia-prone states and should not be interpreted as a target.

Units

%, mmol/mol, % of total Hb

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.