Direct measurement of progesterone in plasma and milk by a simple solid-phase radioimmunoassay

A rapid method for determination of progesterone in bovine, goat and porcine plasma as well as in bovine milk was evaluated. The method employed was a solid-phase (125)I radioimmunoassay equipped with progesterone standards in human serum and called the Coat-A-Count procedure. The dilution curves of bovine plasma samples with high progesterone content were parallel with the standard curve based on human serum. The relation between measurements of progesterone levels in bovine plasma using the reference extraction method and the direct Coat-A-Count procedure was highly significant, resulting in the linear regression equation Y = 1.06x-0.04. In case of goat and porcine plasma, the direct method yielded higher results than the reference extraction method (Y = 1.37x + 1.38 and Y = 1.69x - 6.47, respectively). Progesterone concentration in bovine whole milk was much higher when measured by the Coat-A-Count procedure than by the reference Farmos kit (Y = 1.59x + 1.51). However, when the same samples were assayed by a modified Coat-A- Count procedure, i.e. progesterone standards from Farmos kit, the values were more or less identical (Y = 0.88x - 0.21).