Measurement of basal growth hormone (GH) is a useful test of disease activity in treated acromegalic patients

Abstract

Nadir GH during oral glucose tolerance test (OGTT) is the gold-standard test of GH secretion in treated acromegaly. However, it was recently reported that variability in GH is reduced postradiotherapy, making basal GH a potential surrogate marker for nadir GH in such patients. We aimed to investigate how predictive basal GH is of nadir GH and IGF-I, and whether radiotherapy influenced these relationships. A total of 226 pairs of basal and nadir GH values from 76 treated acromegalic patients were analysed. Basal GH was defined as the fasting serum GH immediately prior to OGTT. A highly positive linear correlation (Pearson correlation = 0.955, P < 0.01) between basal and nadir GH was found. Negative predictive value for basal GH < 1 microg/l with respect to nadir GH > 1 microg/l was 100% (53/53 in radiotherapy group, 15/15 in nonradiotherapy group). Positive predictive values for basal GH > 2 microg/l with respect to nadir GH > 1 microg/l for patients treated and not treated with radiotherapy were 96.7% (88/91) and 95.2% (20/21), respectively. No significant difference between concordance of basal and nadir GH with IGF-I in assessment of disease activity was found. Discordance between IGF-I and nadir or basal GH < 1 microg/l was lower in the radiotherapy group than nonradiotherapy group, but this was nonsignificant. Basal GH < 1 microg/l and > 2 microg/l are highly predictive of nadir GH < 1 microg/l and > 1 microg/l, respectively, regardless of previous radiotherapy. Basal GH is as good as nadir GH in concordance with IGF-I. We therefore suggest basal GH is a useful test of disease activity in treated acromegaly, and can reliably replace OGTT unless basal GH is between 1 microg/l and 2 microg/l.