Disposition index (DI) and glucose effectiveness (S_G) are risk factors for diabetes. However, the effect of DI and S_G on future diabetes has not been examined in large epidemiological studies using direct measures.

Insulin sensitivity index (S_I), acute insulin response (AIR), and S_G were measured in 826 participants (aged 40–69 years) in the Insulin Resistance Atherosclerosis Study (IRAS) by the frequently sampled intravenous glucose tolerance test. DI was expressed as S_I × AIR. At the 5-year follow-up examination, 128 individuals (15.5%) had developed diabetes.

The area under the receiver operating characteristic curve of a model with S_I and AIR was similar to that of DI (0.767 vs. 0.774, P = 0.543). In a multivariate logistic regression model that included both DI and S_G, conversion to diabetes was predicted by both S_G (odds ratio × 1 SD, 0.61 [0.47–0.80]) and DI (0.68 [0.54–0.85]) after adjusting for demographic variables, fasting and 2-h glucose concentrations, family history of diabetes, and measures of obesity. Age, sex, race/ethnicity, glucose tolerance status, obesity, and family history of diabetes did not have a significant modifying impact on the relation of S_G and DI to incident diabetes.

The predictive power of DI is comparable to that of its components, S_I and AIR. S_G and DI independently predict conversion to diabetes similarly across race/ethnic groups, varying states of glucose tolerance, family history of diabetes, and obesity.