Comparison of a solid-phase, no-extraction radioimmunoassay for progesterone with an extraction assay for monitoring luteal function in the mare, bitch, and cow

Abstract Equine, canine, and bovine plasma samples were assayed for progesterone (P 4 ) using extracted plasma in a liquid-phase assay and unextracted plasma in a solid-phase 125 I direct assay developed for human serum. The direct assay was also used to monitor P 4 levels in defatted milk. Results indicated that the direct-assay method was as reliable as the extraction assay for monitoring changes in plasma P 4 during the estrous cycle and early pregnancy. The regression equation specifying the relationship for the two methods was exponential. In this study, correlation coefficients from data subsets ranged from 0.93 to 0.99. The direct assay for P 4 did result in higher values than the extraction assay in plasma from diestrous animals. The use of diluted standards with stripped plasma from the species being assayed gave values that correspond more closely to the extraction assay. The comparisons in this study indicate that the direct radioimmunoassay (RIA) method offered a convenient, reliable method for monitoring luteal function in the species that were evaluated.