Glucose tolerance is the body’s ability to handle a surge of glucose — to clear it from the blood into tissues without letting blood sugar climb too high or stay elevated too long. It is measured formally by the oral glucose tolerance test (OGTT): after an overnight fast, the patient drinks a standard glucose solution and blood sugar is sampled over two hours. A healthy response peaks modestly and returns to baseline; impaired tolerance or a diabetic response — fasting or two-hour values above set thresholds — signals failing insulin action. The test diagnoses diabetes, gestational diabetes, and pre-diabetes, and it remains the reference standard against which simpler fasting tests are judged.