GTT short for Glucose Tolerance Test is a test designed to assess the body response to glucose. In GTT, the patient is given a glucose solution and blood samples are drawn afterword at intervals to measure how well the body cells are able to absorb glucose. There are several variations to the glucose tolerance test used in different conditions but, the most common one of them is the Oral glucose tolerance test or OGTT.

The OGTT is mainly used in the diagnosis of gestational diabetes. For OGTT the patient is required to fast for 8 hours and then a fasting plasma glucose is tested, after that oral glucose solution is given. After that blood samples can be drawn up to 4 times at different intervals to measure the blood glucose. A OGTT is usually performed in the morning as glucose levels usually fall by afternoon.

OGTT used to be the gold standard in the diagnosis of diabetes type 2 but, is now being replaced with other GTT methodology. The GTT is primarily used for the diagnosis of diabetes, insulin resistance, impaired beta cell function, carbohydrate metabolism disorder and also reactive hypoglycaemia and acromegaly.