Discrimination, adjusted correlation, and equivalence of imprecise tests: application to glucose tolerance

Comparison studies between physiological tests are often unsatisfactory for assessing their ability to distinguish between subjects. We recommend a simple but comprehensive protocol, using duplicate testing, that compares tests using 1) the discriminant ratio (DR) between the underlying between- and within-subject SDs, 2) correlation coefficients adjusted for attenuation due to test imprecision, and 3) unbiased estimation of the underlying linear relationship between test results. The following five alternative methods for assessing glucose tolerance were compared: fasting plasma glucose (FPG) as a single sample or as the mean of three 5-min samples (FPG3); the 1- and 2-h glucose during a low-dose intravenous glucose infusion (CIG); and the 2-h plasma glucose from a 75-g oral glucose tolerance test (OGTT). All tests had similar DRs ranging from 2.6 to 4.2. The adjusted correlation between FPG and CIG tests approached unity, and those between OGTT and other tests were approximately 0.9, showing that FPG3 provides similar information to the OGTT. FPG concentrations of 6.0 and 7.1 were found equivalent to the 1985 World Health Organization OGTT thresholds for impaired glucose tolerance and diabetes (7.8 and 11.1 mmol/l).