A single reagent thyroxine radioimmunoassay

An homogenous radioimmunoassay of thyroxine using labelled antigen antibody complex as a single reagent is described. Variation of dose response curve slope with assay incubation time has been studied at two different antisera concentrations. The dissociation constant of the complex was found to be 3.5·10⁻⁴ s⁻¹. The assay involves only three pipettings and requires 45 minutes incubation at 37 °C and 10 μl of serum. Though the assay is essentially a nonequilibrium type, apparent increase in sample values, due to the drift of dose response curve for 45 minutes delay was less than 7.5%. Within and between assay variations were both less than 5% c. v. Assay has been validated by recovery experiments. Analysis of 40 serum samples by the proposed method and by an equilibrium assay gave similar values; r=0.957, Y=0.955X+0.478.