THE CONTINUOUS LOW-DOSE INSULIN AND GLUCOSE-INFUSION TEST - A SIMPLIFIED AND ACCURATE METHOD FOR THE EVALUATION OF INSULIN SENSITIVITY AND INSULIN-SECRETION IN POPULATION STUDIES

In this study we investigated a simple nonlabor-intensive method to ev aluate insulin sensitivity and beta-cell function which is suitable fo r application in population studies. The method is a refinement of the modified Harano test and consists of a continuous low dose insulin (2 5 mU/kg.h) and glucose (4 mg/kg.min) infusion test (LDIGIT) lasting 15 0 min. Insulin sensitivity was evaluated as the MCR of glucose divided by the steady state serum insulin level achieved at the end of the te st. Insulin secretion was expressed as the incremental area for C-pept ide concentration during the first 15 min of the test. We compared the indices of insulin sensitivity and insulin secretion yielded by LDIGI T with those derived from the euglycemic clamp and the hyperglycemic c lamp, respectively. Fifty-four subjects underwent a LDIGIT (33 with no rmal glucose tolerance and 21 with impaired glucose tolerance); of the 54, 19 were submitted to a euglycemic clamp, 18 to a hyperglycemic cl amp, and 10 to a modified Harano test (insulin infusion, 50 mU/kg.h; g lucose infusion, 6 mg/kg.min). LDIGIT overcame the drawbacks associate d with the modified Harano test because it resulted in more stable fin al glucose levels and prevented the occurrence of hypoglycemic episode s. No significant differences were found between the insulin sensitivi ty index (ISI) of the LDIGIT and that of the euglycemic clamp for each group of subjects. Moreover, there was a strong correlation between t he ISI determined by LDIGIT and the ISI determined by clamp (r = 0.90; P < 0.0001), and the best regression line was not different from the identity line, suggesting that the two indices are equivalent. The ind ex of insulin secretion provided by LDIGIT correlated well with that o f the hyperglycemic clamp (r = 0.82; P < 0.001) and was significantly higher in overweight subjects than in normal weight subjects. In concl usion, LDIGIT is a simple and accurate method to assess insulin sensit ivity and secretion. It can be useful in population studies and in sit uations when more complex techniques are not feasible.