Solid-phase, magnetic particle radioimmunoassay.

A solid-phase radioimmunoassay system has been developed based on the use of antibodies covalently linked to polymer-coated iron oxide (EnzacrylR). An electro-magnet is employed both to mix the particles during incubation (by switching the field on and off) and to separate the antibody-bound and free fractions. This obviates the need for vertical rotation and for the time-consuming, multiple centrifugations required with conventional solid phase procedures. The system is universally applicable and methods have been established for the assay of thyroxine, human placental lactogen and digoxin. The thyroxine assay was employed as a model and it was shown that the results obtained for serum samples correlated closely with those using a routine liquid-phase radioimmunoassay. The applicability of employing a second antibody linked to the iron oxide particles was also studied.