Growth hormone (GH) assays: influence of standard preparations, GH isoforms, assay characteristics, and GH-binding protein

The impact of the adoption of the new biosynthetic growth hormone (GH) WHO International Reference Preparation (IRP 88/624), and the recommendation to report results in microgram/L instead of mU/L, is described. Conversion factors were determined by comparing both the linear and nonlinear relations of the GH values. The Pharmacia polyclonal IRMA (p-IRMA) and the DELFIA monoclonal time-resolved immunofluorometric assay (trIFMA) with kit calibrators calibrated either against the pituitary-derived WHO IRP 80/505 or the new 88/624 were evaluated. Conversion factors of 4.17 mU/L = 1 microgram/L for the p-IRMA and 4.31 mU/L = 1 microgram/L for the trIFMA were necessary. Different cross-reactivity patterns for the deaminated and dimer 22-kDa, 20-kDa, and 17-kDa GH isoforms were found. Expected GH recovery was similar when the measured values were adjusted according to the results of the cross-reactivity study.