A simple and rapid solid-phase radioimmunoassay for serum progesterone, using the protein A of Staphylococcus aureus...

A simple, rapid, and inexpensive radioimmunoassay method for serum progesterone is described, which uses a solid-phase technique for separation of antibody-bound from antibody-free progesterone. Rabbit antiprogesterone immunoglobulins are adsorbed on the protein A of formaldehyde- and heat-treated Staphylococcus aureus cells (Pansorbin; Calbiochem-Behring Corp., La Jolla, Calif.). The suspension of antibody-coated pansorbin retains all its binding activity (23% for 500 microliter of a 0.033% suspension) of 1-2-H(N)-progesterone (20,000 cpm in 100 microliter) when kept at +4 degrees or at -25 degrees C for at least 4 months. Dose-response curves obtained with ether-serum extracts and with the progesterone standard do not deviate significantly from parallelism. The progesterone standard gives identical dose-response curves whether diluted in the assay buffer or in a progesterone-free ether-serum extract. The sensitivity of the assay is 0.02 ng/assay tube. The intra-assay variation coefficient is 16%, and the routine interassay variation coefficient is 17%. The mean serum progesterone concentrations were 0.55 ng/ml during the follicular phase of the menstrual cycle and 12.5 ng/ml during the luteal phase. The average blank value for distilled water was 0.02 ng/assay tube.