INTRAINDIVIDUAL VARIATION OF GLUCOSE, SPECIFIC INSULIN AND PROINSULINCONCENTRATIONS MEASURED BY 2 ORAL GLUCOSE-TOLERANCE TESTS IN A GENERAL CAUCASIAN POPULATION - THE HOORN STUDY

We studied the intra-individual variation in plasma glucose, specific serum insulin and serum proinsulin concentrations, measured by two 75- g oral glucose tolerance tests in an age, sex, and glucose tolerance s tratified random sample from a 50-74-year-old Caucasian population wit hout a history of diabetes mellitus. The intra-individual variation wa s assessed by the standard deviation of the test-retest differences (S Ddif). For subjects with normal (n = 246), impaired glucose tolerance (n = 198), and newly detected diabetes (n = 80) classified at the firs t test, the following (SDdif/median level of individual average scores ) were found: fasting glucose: 0.4/5.4, 0.5/5.9 and 0.7/7.2 mmol/l; 2- h glucose: 1.3/5.6, 1.8/8.5 and 2.3/12.8 mmol/l; fasting insulin: 23/7 6, 32/89 and 30/ 116 pmol/l; 2-h insulin: 190/303, 278/553 and 304/626 pmol/l; fasting proinsulin: 4/8, 6/13 and 9/18 pmol/l; 2-h proinsulin : 19/49, 23/84 and 33/90 pmol/l: respectively. In both glucose, proins ulin and insulin concentrations the total intra-individual variation w as predominantly determined by biological variation, whereas analytica l variation made only a minor contribution. The SDdif can easily be in terpreted, as 95% of the random test-retest differences will be less t han 2 . SDdif, or in terms of percentage, less than (2 . SDdif/median level of individual average scores). 100, Therefore, for subjects with normal glucose tolerance, 95% of the random test-retest differences w ill be less than 15% (fasting glucose), 46% (2-h glucose), 61% (fastin g insulin), 125% (2-h insulin), 100% (fasting proinsulin) and 78% (2-h proinsulin) of the median value of the individual average scores. No substantial independent association of either age, gender or obesity w ith the intra-individual variation in glucose, proinsulin, or insulin concentrations was found.