Evaluation of the reproducibility of the GHRH plus GHRP-6 test of growth hormone reserve in adults

BACKGROUND: The diagnosis of GH deficiency (GHD) is controversial, relying on GH secretion elicited by the provocative tests of GH reserve. However, the performance of most of the tests in use have not been evaluated rigorously. The repeatability of a test is a prerequisite before evaluating its diagnostic capability. OBJECTIVE: The combined administration of GH-releasing hormone (GHRH) and GH-releasing hexapeptide (GHRP-6) is an efficacious test of GH reserve in adults, and the target of this work was to evaluate its reproducibility. METHODS: Seventeen healthy subjects were challenged with GHRH plus GHRP-6 (1 micro g/kg i.v.). All subjects underwent four tests on different days separated by at least 2 months. GH peaks were evaluated by several mathematical analyses of reproducibility. RESULTS: As a group, the subjects showed high reproducibility after the four tests, with GH peaks of 46.0 +/- 5.1; 48.4 +/- 6.4; 50.1 +/- 5.4 and 52.9 +/- 5.8 micro g/l, respectively (1 micro g/l = 3 mU/l). Individually analysed, the reproducibility was good, and the regression analysis showed a correlation between tests 1 and 2 of R = 0.729, P < 0.0009, between tests 1 and 3 of R = 0.710, P < 0.001, and between tests 1 and 4 of R = 0.683, P < 0.002. Under mathematical analysis, the multiple correlation coefficient analysis, analysis of variance (anova) with repeated measurements, repeatability index, the simplest coefficient of variation and the intraclass correlation coefficient (ICC) all unambiguously showed that the GHRH + GHRP-6 test was reproducible. Furthermore, the repeated tests did not alter the biochemical diagnosis of the subjects, with absence of false-positive values. CONCLUSIONS: The GHRH + GHRP-6 test of GH reserve is reproducible in adult subjects.