oral glucose tolerance test

The oral glucose tolerance test is a standardized challenge that asks the body to clear a known sugar load, revealing how well its insulin system copes. After an overnight fast, the person drinks a measured glucose solution — typically 75 grams dissolved in water — and blood glucose is checked at the start and again two hours later. It is a stress test for glucose control rather than a single resting reading.

In a healthy person, the surge of glucose triggers a brisk release of insulin that pulls sugar out of the blood, so the two-hour value falls back into a normal range. If the pancreas cannot make enough insulin, or the body resists insulin's action, glucose lingers high. The two-hour reading therefore grades a spectrum: normal tolerance, impaired (prediabetes), or diabetes, using defined cutoffs.

The OGTT is more sensitive than a fasting glucose alone for catching early glucose problems, but it is more burdensome — fasting, sitting still for two hours, occasional nausea from the sweet drink — and its result can vary with diet, illness, and activity in the preceding days. It is especially important for screening gestational diabetes in pregnancy, often using a different glucose dose and timed samples.