HbA1c, fasting glucose, random glucose and the tolerance test
Expansion
| Test | Diabetes | Prediabetes or high risk |
|---|---|---|
| HbA1c | 48 mmol/mol (6.5 per cent) or above | 42 to 47 |
| Fasting glucose | 7.0 mmol/l or above | 6.1 to 6.9 (impaired fasting glucose) |
| 2 hour OGTT | 11.1 mmol/l or above | 7.8 to 11.0 (impaired glucose tolerance) |
| Random glucose | 11.1 or above with symptoms |
Confirmation: with symptoms (thirst, polyuria, weight loss) a single abnormal result suffices. Without symptoms, a second confirmatory test on a different sample is required.
When HbA1c cannot be used
- Anything that alters red cell lifespan: haemolysis, recent transfusion, haemoglobinopathies (sickle, thalassaemia), splenectomy, iron or B12 deficiency and their treatment, erythropoietin
- Rapid onset diabetes: type 1, children and young people, symptoms under 2 months, drug induced (steroids, antipsychotics), pancreatic damage, acute illness
- Pregnancy and the postpartum period
- Chronic kidney disease stage 4 or 5, and HIV treatment
In these situations use glucose based criteria.
Gestational diabetes has separate thresholds: fasting 5.6 mmol/l or above, or 2 hour 7.8 or above on a 75 g oral glucose tolerance test, typically at 24 to 28 weeks in those with risk factors.
Type 1 versus type 2 where the picture is unclear: ketones, C-peptide (low in type 1), and autoantibodies (GAD, IA-2, ZnT8). Consider MODY in a young, non-obese patient with a strong autosomal dominant family history and negative antibodies, since treatment may be sulfonylurea rather than insulin.