Glucose tolerance test involves drinking a glucose solution, and measuring glucose concentration thereafter through subsequent blood sample(s). It determines how quickly it is cleared from the blood. In diabetes, they tend to remain high. It is used to test for insulin resistance and diabetes.

• OGTT is a done in the morning after an overnight fast of between 8-14 hours. During the previous 3 days, the subject must have an unrestricted diet containing at least 150g carbohydrates per day, and unlimited physical activity. The test involves drinking a solution of 75 or 100g glucose, and blood glucose is measured at the start, and on set intervals after (1, 2, 3 hours after). There is weak agreement between the 75 and 100g test, and the 75g will identify fewer Pt's at risk.
• GCT (aka O'Sullivan test) is a 50g version of the OGTT, and requires no fasting, and involves drinking a solution of 50g glucose, and blood sugar is measured 1 hour later. 

The distinction betwen pre-diabetes and diabetes, is that pre-dibetes has macro but not microvascular risk, and diabetes has both macro and microvascular risk

Source: 2012/june/oral-glucose-tolerance-testing/">RACGP

• The glucose solution is very sweet and some women find it unpleasant, therefore, sometimes, artificial flavors are added
• Pt's may experience nausea during the test, particularly with ingesting higher glucose levels

• Diabetes
• Gestational diabetes