Somatotropin as measured by a two-site time-resolved immunofluorometric assay

To date, many of the current criteria for diagnosis of somatotropin (growth hormone, GH) deficiency have been based upon measurement of this hormone by competitive radioimmunoassay (RIA) with use of polyclonal antibodies. In recent years, however, the development of hybridoma technology has led to the generation of various monoclonal antibodies (Mabs) to GH with different affinities and epitope specificities. Subsequently, these reagents have been used in the development of noncompetitive two-site immunometric assays (e.g., immunoradiometric assay; IRMA). In general, the values obtained for serum GH by IRMA have been lower than those obtained by RIA, because of the epitope-specificity profile of the Mabs in the IRMA. Attempting to obtain GH values numerically similar to those by RIA, we used a combination of Mabs to GH in developing and evaluating a two-site time-resolved immunofluorometric assay (IFMA) based on the streptavidin-biotin interaction. Fluorescence is proportional to concentration of analyte and is linearly related to concentration over the range 0.3 to 40 micrograms/L. The assay was satisfactory with respect to sensitivity, accuracy, and precision (CV less than 10% over the entire working range). In addition, the concentration of GH was determined by the IFMA and a competitive RIA in serum obtained from GH deficient and acromegalic patients. The pairing of antibodies in the IFMA gave numerical values that agreed well with those by RIA (r = 0.97; n = 100).