Insulin sensitivity indices calculated from basal and OGTT-induced insulin, glucose, and FFA levels

Insulin Sensitivity Indices for glycemia [ISI(gly)] and blood FFA [ISI(ffa)] can be calculated with the formulas: ISI(gly) = 2/[(INSp x GLYp) + 1], and ISI(ffa) = 2/[(INSp x FFAp) + 1], where INSp, GLYp and FFAp = insulinemic, glycemic, and FFA areas during OGTT (75 g glucose) of the person under study, simplified by considering only data at 0 and 2 h (0-2 h areas), according to WHO criteria or, better, at 0, 1 and 2 h (0-1-2 h areas). Expressed as unit/ volume.h-1, 0-1-2 h area is equal to 1/2 value at 0 min + value at 1 h + 1/2 value at 2 h, while 0-2 h area is equal to value at 0 + value at 2 h. Instead of areas, basal levels can also be used. Basal levels and areas are expressed taking the mean normal value as unit, so that in normal subjects ISI(gly) and ISI(ffa) are always around 1, with maximal variations between 0 and 2. Each laboratory should have its normal reference values for basal levels and OGTT areas. However, reliable mean normal values were selected from literature. Based on meta-analysis of published data, ISI(gly) and ISI(ffa) were reduced in subjects who were overweight and/or IGT and in NIDDM patients and their relatives. Moreover, correlation of ISI(gly) with the euglycemic clamp data was significant. However, it should be stressed that the clamp procedure is performed under artificially induced steady-state whereas ISI(gly) and ISI(ffa) are obtained under rather physiological conditions, with hormonal and metabolic variables unmodified, thus being suitable to assess whole-body insulin sensitivity in the clinical setting.