IVGTT

The intravenous glucose tolerance test (IVGTT) is used to evaluate islet function in vivo. It does not have greater diagnostic value than the OGTT (oral glucose tolerance test) in the diagnosis of diabetes mellitus so is used only in research. It measures the early insulin response following an intravenous glucose load, and is useful in transplantation studies and in prediabetic individuals with elevated islet cell antibodies and other autoimmune markers.

The classic minimal model of glucose kinetics (Bergman RN, Finegood DT, Ader M. Assessment of insulin sensitivity in vivo. Endocrine Reviews 1985; 6:45-86.) is widely used to investigate glucose metabolism in vivo in physiological, pathological and epidemiological studies. The standard IVGTT consists in injecting, say at time 0, glucose over a period of 30-60 seconds and measuring in plasma the resultant glucose and insulin concentrations. The sampling schedule of the standard IVGTT usually consists of 3 pretest samples taken at -15, -5 and 0 (immediately before the injection) and 25 test samples taken at 2, 3, 4, 5, 6, 8, 10, 12, 15, 18, 20, 25, 30, 35, 40, 60, 70, 80, 100, 120, 140, 160, 180, 210, 240 minutes.

The following figure shows results of the IVGTT of a normal (nondiabetic) as the red line with round points. (Click for a larger view.) The yellow and brown lines show abnormal IVGTT's obtained in one of our experiments.

K Value

The results of an IVGTT are reported as a K Value, a reflection of the time required for glucose to clear the circulation.

K = 0.69/T1/2 X 100, where T1/2 is the time required for plasma glucose to reach one half of the calculated zero-time concentration. The normal value for K is 1.2 or greater.