Evaluation of two highly sensitive assays for serum IGF-1 and GH determination following oral glucose tolerance test in healthy controls.

Objective: To evaluate 2 highly sensitive assays for serum insulin-like growth factor-1 (IGF-1) and growth hormone (GH) determination following an oral glucose tolerance test (OGTT) in healthy controls.

Methods: Nineteen healthy adults underwent a standard 75g OGTT and GH and IGF-1 were measured. Serum GH and IGF-1 levels were assayed by a sensitive immunoradiometric assay (IRMA) and a highly sensitive chemiluminescent immunometric assay (CLIA).

Results: The mean IGF-1 concentration was 153±65ng/ml measured by IRMA and 144±56ng/ml measured by CLIA. The median (interquartile range) basal GH concentrations by IRMAvs CLIA were 0.8 (0.5-3) μg/l vs 0.5 (0.1-2.4) μg/l. The median nadir GH measured by IRMA in these subjects was 0.4 (0.3-0.5) μg/l, and the mean nadir GH by CLIA was 0.08 (0.01-0.22) μg/I. When a ratio of basal IRMA/CLIA GH was calculated in each subject, the median ratio of basal IRMA/CLIA GH concentrations in subjects overall was 1.68. Similarly, the median ratio of nadir IRMA/CLIA GH values was 4.44. One of the subjects did not achieve GH suppression into the established normal range, with a GH nadir of 1.2 μg/l by IRMA and 1 μg/l by CLIA, overlapping with the traditional cut-off defining acromegaly when GH suppression was measured by IRMA.

Conclusions: Highly sensitive chemiluminescent immunometric assays should be used to assess the GH/IGF-1 axis. In our opinion, there is no need for a lower GH suppression cut-off for diagnosing acromegaly. We found no significant gender-, BMI- or age-related differences in nadir GH levels and thus our results do not support different OGTT criteria for screening of acromegaly in men and women, or in younger and older subjects.