Evaluation of the clinical utility of the ultrasensitive immunofluometric assay for growth hormone (GH) and of the cortisol secretory pattern in prediction of the linear growth response to treatment with GH.

OBJECTIVES: To evaluate the utility of an ultrasensitive IFMA for human 22 kDa GH in assessment of GH secretion and prediction of the linear growth response to exogenous GH.
METHODS: Utilizing Delfia reagents supplied by Wallac-OY, an ultrasensitive IFMA for GH was established. Serum GH concentrations from 15 children/adolescents undergoing 24 hour GH secretory profiles with sampling at 20 minute intervals were analyzed by both IFMA and RIA. Cortisol values were also measured. Twelve children were later treated with GH. The 24 hour GH and cortisol secretory profiles were analyzed by the Cluster program and the relationships of these profiles to the linear growth response to exogenous GH determined.
RESULTS: The sensitivity of the IFMA for GH relative to a zero standard was 0.005 ng/ml; intra-assay coefficients of variation ranged from 12% at a GH concentration of 0.005 ng/ml to 4% at 0.038 ng/ml; interassay coefficients of variation ranged from 34% at a GH concentration of 0.005 ng/ml to 10.5% at 2.7 ng/ml and to 2.7% at 12.7 ng/ml. Above assay sensitivity, there was good correlation between GH concentrations determined by IFMA and those by IRMA and RIA (r = 0.998 and 0.992 respectively). The number of GH secretory peaks identified by IFMA was significantly greater than that detected by RIA (10.6 +/- 3.2 [SD] vs 6.7 +/- 3.3/24 hours, p = 0.0001 by paired t-test). There were few significant relationships between any parameter of GH secretion measured by RIA or IFMA (peak GH pulse amplitude, percent increase in amplitude, area under the peak, interpeak interval) and the pretreatment growth rate, the growth velocity while receiving GH therapy, or the increment in growth rate during administration of GH. The number of GH secretory peaks determined by RIA correlated weakly with the pretreatment growth rate. There was no meaningful relationship between the serum concentrations of cortisol and GH-IFMA. Peak GH concentrations and nadir cortisol values were exactly coincident in 15.7% (25/159); 42.8% of nadir cortisol values coincided with or were within +/- 20 minutes of peak GH values (68/159). However, there was no relationship between the number of cortisol secretory peaks, the pooled 24 hour and nocturnal concentrations of cortisol and the pretreatment growth velocity, the growth rate or increment in growth velocity during administration of GH.
CONCLUSIONS: Despite the increased sensitivity of the IFMA and its ability to detect pulsatile GH secretion heretofore unidentified, data from this GH assay were not useful in predicting first year growth rate during administration of GH. The secretory pattern of cortisol was not helpful in predicting the growth response to GH.