OGTT is one of the tests that can be used to diagnose diabetes mellitus or impaired glucose tolerance (IGT), and it is useful for a better diagnostic and prognostic assessment, especially in subjects with impaired fasting glucose (IFG), because a significant proportion of these subjects have a glucose response compatible with the diagnosis of diabetes.

The OGTT, an examination with low reproducibility, must be performed correctly and standardized. Two samples are collected, the first on an empty stomach and the second 2 h after oral intake of a glucose solution (300 mL) containing anhydrous glucose (75 g in adults and 1.75 g per kg of body weight in children, up to a maximum of 75 g).

Preanalytical Aspects

The test should not be performed in subjects with a documented history of diabetes mellitus or with investigations indicating the presence of diabetes mellitus; it should not be performed during acute pathologies and in the presence of factors that may influence the test itself, such as in the presence of recent infections, during convalescence from surgery, or during treatment with life-saving drugs; and finally, it should not be performed in gastro-resected subjects because of the risk of evoking the symptoms of the syndrome of impaired gastric emptying.

In the 3 days preceding the test, the subject should consume a standardized amount of carbohydrates (100–150 g per day) without changing the diet. The intake of nonlife- saving drugs (e.g., thiazides, contraceptives, and corticosteroids) should be postponed until after the OGTT has been performed.

The examination should be carried out on an empty stomach (at least 8 h and not more than 14 h).

The speed of intake of the glucose drink, administered at a temperature of 15–25 °C, should not exceed 5 min.

During the test, the subject must remain seated and is not allowed to drink, eat, or smoke. The test must be performed in the morning to avoid alterations in glycemia due to the previously mentioned circadian variability.

Throughout the procedure, the subject must remain in an area under the control of medical personnel, who can intervene in cases of illness and possibly decide to suspend the examination. The examination is not valid if vomiting occurs.

Analytical Aspects

Plasma blood glucose is measured using the methods described in section “Fasting blood glucose.”

Postanalytical Aspects

 As pointed out in section “Fasting blood glucose,” basal and afterload blood glucose values during OGTT should be expressed in mmol/L. However, many laboratories still express blood glucose in mg/dL. Plasma glucose reading 2 h after OGTT of less than 7.8 mmol/L (140 mg/dL) is considered normal. If the value obtained is between 7.8 and 11.0 mmol/L (respectively 140 and 199 mg/dL), the diagnosis of IGT is made.

Diabetes mellitus is diagnosed (with a second diagnostic criterion met) if blood glucose 2 h after OGTT is greater than or equal to 11.1 mmol/L (200 mg/dL).

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