Objective: To review current practice in centers that use the IVGTT for prediction of IDDM. To establish consensus protocol for performance of the test.
Research design and methods: Postal questionnaires were delivered to 12 centers.
Results: Eleven centers used a glucose dose of 0.5 g/kg and 1 used 0.3 g/kg; the dosage in adults was limited to a maximum of 25-50 g in some centers but others applied no upper limit. The glucose concentration of the infusate varied between 20 and 66%. Eight centers injected glucose manually, two used a syringe pump, and two used gravity infusion. The period of infusion ranged from 30 +/- 10 s to 4 +/- 2 min, and time zero was taken as the start (1 center), middle (1 center), or end (10 centers) of the infusion. The potential range in timing of the +1-min sample varied between 1 and 7 min from the start of the infusion. Quality-assurance standards for the insulin assays used were not always appropriate for the fasting and low stimulated range of insulin levels.
Conclusions: The first-phase insulin response to the IVGTT is widely measured as an index of risk of progression to IDDM. We established that methodology varies widely. Because of this, a new standard protocol for use in prediction of IDDM was agreed by an ICARUS working group and is described herein.