Modified solid-phase (tube) radioimmunoassay of human growth hormone

Polystyrene tubes were coated with rabbit antiserum to human growth hormone (HGH). Bovine serum albumin (BSA) in buffer, HGH standard or unknown samples, and 125I-HGH were added to the antibodycoated tubes. After incubation the radioactivity in each tube was counted, the incubate discarded, the tubes were washed, and radioactivity was recounted. The sensitivity of the assay was consistently 0.05 mμg HGH or less. Nonspecific binding by uncoated tubes was less than 1%. It was preferable to control the surface area exposed to excess antiserum rather than limit the concentration of the antiserum. Polypropylene and polystyrene tubes gave similar results. Variation of ionic composition or pH of the incubate had no significant effects. Maximum precision and sensitivity occurred with incubation at 4 °C for 40 to 60 h. Addition of posthypophysectomy human plasma or varying the protein concentration had similar effects. The immobilized antibody may be more selective in binding with antigen than is free antibody in solution. Our modifications combine favorable features of the solid-phase system with an increase in sensitivity and greater economy of materials.