Interpreting HbA1c diagnostic tests
For the diagnostic thresholds of tests to diagnose diabetes, see Diagnostic thresholds for diabetes.
A venous blood sample (taken and analysed at an accredited laboratory) for glycated haemoglobin (HbA1c) giving a result of 48 mmol/mol (6.5%) or more is diagnostic of diabetes. However, an HbA1c less than 48 mmol/mol (6.5%) does not refute a diagnosis of diabetes made based on venous blood glucose concentrations or an oral glucose tolerance test. HbA1c values more than 48 mmol/mol (6.5%) are associated with microvascular complications of diabetes (retinopathy, diabetic kidney disease and neuropathy).
Australian guidelines recommend that an HbA1c of 42 to 46 mmol/mol (6.0 to 6.4%) is consistent with a diagnosis of prediabetes in adults1 (see Diagnostic thresholds for diabetes for prediabetes thresholds). Patients with an HbA1c in this range may also have prediabetes defined by impaired fasting glucose or impaired glucose tolerance, or may have diabetes defined by other diagnostic tests. For asymptomatic patients with an HbA1c in the range for prediabetes:
- further testing with either an oral glucose tolerance test or fasting venous blood glucose concentration can detect patients who have diabetes
- further testing with an oral glucose tolerance test can be considered if the diagnosis of impaired glucose tolerance will affect management decisions (eg whether to start general lifestyle modification or a structured, intensive lifestyle intervention—see Management of people at risk of developing type 2 diabetes).