This test is performed to evaluate the horse's ability to metabolize glucose normally. Insulin is a hormone that is critically involved in blood sugar (glucose) regulation. Increases in insulin cause a decrease in levels of blood sugar. Insulin resistance is the body's failure to respond to insulin in a normal way and is an extremely common condition in domestic horses. Type 1 diabetes is the lack of insulin, and is very rare in horses.

When a horse is presented with a sugar (glucose) load in the blood, it should produce a spike of insulin, which then causes the sugar to drop in a predictable way. The test requires fasting the horse for 8 hours before it is performed. Sugar (dextrose) is given to the horse intravenously. Blood samples are taken at specific times up to 4 hours post injection. Blood glucose levels should be normal in 3 hours after injection. High blood glucose remaining at 3 hours indicates insulin resistance or type 1 diabetes.