Diagnosis
Diabetes mellitus is diagnosed with a test for the glucose content in the blood, and is diagnosed by demonstrating any one of the following:
- Fasting plasma glucose level ≥ 7.0 mmol/L (126 mg/dL). For this test, blood is taken after a period of fasting, i.e. in the morning before breakfast, after the patient had sufficient time to fast overnight or at least 8 hours before the test.
- Plasma glucose ≥ 11.1 mmol/L (200 mg/dL) two hours after a 75 gram oral glucose load as in a glucose tolerance test (OGTT)
- Symptoms of high blood sugar and plasma glucose ≥ 11.1 mmol/L (200 mg/dL) either while fasting or not fasting
- Glycated hemoglobin (HbA1C) ≥ 48 mmol/mol (≥ 6.5 DCCT %).
| Condition | 2-hour glucose | Fasting glucose | HbA1c | |||
|---|---|---|---|---|---|---|
| Unit | mmol/L | mg/dL | mmol/L | mg/dL | mmol/L | mg/dL |
| Normal | < 7.8 | < 140 | < 6.1 | < 110 | < 42 | < 6.0 |
| Impaired fasting glycaemia | < 7.8 | < 140 | 6.1–7.0 | 110–125 | 42–46 | 6.0–6.4 |
| Impaired glucose tolerance | ≥ 7.8 | ≥ 140 | < 7.0 | < 126 | 42–46 | 6.0–6.4 |
| Diabetes mellitus | ≥ 11.1 | ≥ 200 | ≥ 7.0 | ≥ 126 | ≥ 48 | ≥ 6.5 |
A positive result, in the absence of unequivocal high blood sugar, should be confirmed by a repeat of any of the above methods on a different day. It is preferable to measure a fasting glucose level because of the ease of measurement and the considerable time commitment of formal glucose tolerance testing, which takes two hours to complete and offers no prognostic advantage over the fasting test. According to the current definition, two fasting glucose measurements at or above 7.0 mmol/L (126 mg/dL) is considered diagnostic for diabetes mellitus.
Per the WHO, people with fasting glucose levels from 6.1 to 6.9 mmol/L (110 to 125 mg/dL) are considered to have impaired fasting glucose. People with plasma glucose at or above 7.8 mmol/L (140 mg/dL), but not over 11.1 mmol/L (200 mg/dL), two hours after a 75 gram oral glucose load are considered to have impaired glucose tolerance. Of these two prediabetic states, the latter in particular is a major risk factor for progression to full-blown diabetes mellitus, as well as cardiovascular disease. The American Diabetes Association (ADA) since 2003 uses a slightly different range for impaired fasting glucose of 5.6 to 6.9 mmol/L (100 to 125 mg/dL).
Glycated hemoglobin is better than fasting glucose for determining risks of cardiovascular disease and death from any cause.
