Journal of Clinical Endocrinology & Metabolism, Vol 75, 1479-1483, Copyright © 1992 by Endocrine Society

ARTICLES

Effect of improved assay sensitivity on luteinizing hormone pulse detection after gonadotropin-releasing hormone antagonist treatment in man

W Salameh, S Bhasin, BS Steiner, M Peterson and RS Swerdloff
Division of Endocrinology, Harbor-University of California-Los Angeles Medical Center, Torrance 90509.

When serum levels of a hormone are at or below the detection limit of the measurement system, accurate characterization and quantitation of pulsatile hormone secretion may be difficult. This point is well illustrated in this study, which used two immunoassays with markedly different assay sensitivities to quantitate pulsatile LH secretion in men in whom serum LH levels had been suppressed by the administration of a GnRH antagonist. Five normal men received 5 mg Nal-Glu, sc, daily for 21 days. Blood was drawn at 10-min intervals over 8 h on days 0 and 21. Samples were assayed in triplicate in both a conventional LH RIA with a sensitivity of 0.6-1.0 IU/L and a two-site-directed ultrasensitive immunofluorometric assay (IFMA) with assay sensitivity ranging from 0.05-0.125 IU/L. LH pulses were analyzed by Cluster analysis (C) and were corroborated by the Detect algorithm (D). Nal-Glu suppressed LH (C) pulse amplitude from 4.0 +/- 0.7 to 0.40 +/- 0.03 IU/L by LH RIA and from 7.8 +/- 2.1 to 0.21 +/- 0.04 IU/L by LH IFMA. An apparent reduction in LH pulse number was observed in the RIA data on day 21 by both pulse detection methods [4.2 +/- 0.4 vs 2.4 +/- 0.2/8 h on days 0 and 21 by C (P < 0.005); 5.8 +/- 0.8 vs. 2.8 +/- 0.7 by D (P < 0.05)]. However, the IFMA measurements in the same samples using the more sensitive LH IFMA showed no difference in pulse number between days 0 and 21. The RIA data correlated well with the IFMA data, with a concordance coefficient ranging from 0.66-0.9. In summary, the Nal-Glu GnRH antagonist markedly decreases LH pulse amplitude, but not pulse frequency. These observations are consistent with competitive inhibition of GnRH action at the pituitary site by the Nal-Glu antagonist; they indicate that assay characteristics can significantly affect quantitation of hormone pulse pattern and underscore the need for ultrasensitive LH assays for accurate assessment of LH pulse characteristics when LH levels are low or suppressed.