Type 2 diabetes is diagnosed from the blood glucose or the HbA1c, and each test has a threshold above which the diagnosis is made. The same thresholds separate diabetes from prediabetes, the intermediate state in which glucose is higher than normal but not yet diabetic.
Diagnostic thresholds
| Category | Fasting plasma glucose | 2-hour plasma glucose (75 g OGTT) | HbA1c |
|---|---|---|---|
| Diabetes | ≥126 mg/dL (7.0 mmol/L) | ≥200 mg/dL (11.1 mmol/L) | ≥6.5% |
| Prediabetes | 100–125 mg/dL (IFG) | 140–199 mg/dL (IGT) | 5.7–6.4% |
| Normal | <100 mg/dL (5.6 mmol/L) | <140 mg/dL (7.8 mmol/L) | <5.7% |
A random plasma glucose of ≥200 mg/dL (11.1 mmol/L) in a person with the classic symptoms of hyperglycemia, or in a hyperglycemic crisis, also establishes the diagnosis.
The fasting plasma glucose is measured after at least 8 hours without calories. The 2-hour value is measured during an oral glucose tolerance test (OGTT), in which the patient drinks a 75 g glucose load after an overnight fast. When an OGTT is used, adequate carbohydrate intake of at least 150 g per day should be ensured for the 3 days before the test, otherwise the glucose response is falsely flattened. HbA1c reports the average glucose over the preceding 2–3 months and needs no fasting, but it can be misleading when red-cell survival is altered, for example in anaemia, haemoglobinopathies, or pregnancy, so glucose tests are used in those situations.
Confirming the diagnosis
In an asymptomatic person, a single abnormal result is not enough: the diagnosis requires two abnormal results, which may come from the same sample or from two different samples. When the person has classic symptoms or presents with a hyperglycemic crisis, a single abnormal result confirms the diagnosis.
Who to screen and how often
Screening is targeted rather than universal. It should be offered to all adults from age 35, and earlier in adults who are overweight or obese and have one or more of the risk factors for type 2 diabetes. The thresholds that define prediabetes are also the thresholds that flag who should be retested.
- Adults with a normal result: repeat screening at least every 3 years, sooner if symptoms appear or risk changes, such as weight gain.
- Adults with prediabetes: test at least yearly, because the yearly risk of progression to diabetes is several percent.
- Adults after gestational diabetes: test at least every 1–3 years.
Finding the disease early matters because hyperglycemia often precedes the diagnosis by years, so vascular damage may already be present at diagnosis.