Objective: To compare the magnitude and reproducibility of the FPIR measured during two different IVGTT protocols in nondiabetic subjects.
Research design and methods: Nine control subjects each had two pairs of IVGTTs with either a 4-min infusion of 0.5 g/kg glucose or a 1-min infusion of 0.3 g/kg glucose. Blood glucose and serum insulin were measured before and 1, 2, 3, 5, and 10 min after completion of the glucose infusion. The FPIR was measured with either 1 + 3-, 2 + 3 + 5-, or 1 + 3 + 5-min serum insulins, areas under the insulin curve (0-5 or 0-10 min), or the ratio of serum insulin to blood glucose area.
Results: The FPIR was higher in eight of nine subjects with the short-infusion test, but the within-subject variation of the two methods was identical. Reproducibility was not significantly improved with an integrated insulin area or insulin-to-glucose ratio measurement.
Conclusions: Reproducibility of the FPIR measured during IVGTT is not significantly affected by the duration of the glucose infusion. However, the magnitude of the difference in FPIR observed between the two protocols highlights the need for standardization of the methodology if the IVGTT is to be used in studies of the preclinical stage of IDDM.