Homogeneous enzyme immunoassay for thyroxine compared with a radioimmunoassay technique

A homogeneous enzyme immunoassay (EMIT) for serum thyroxine determination has been evaluated. It was also compared to our routine method, a radioimmunoassay with use of a commercial kit, Tetra-Tab RIA. Within-day precision was very similar for the two methods, the CV ranging from about 2.5% at 150 nmol of thyroxine per liter to about 5.7% at 25 nmol/L. The two methods were compared by running 100 patients' serum samples. The resulting linear regression equation was: y = 1.088x - 17.9, with a correlation coefficient r = 0.979. The deviation from the theoretical line, y = x, wash shown to be primarily caused by the calibration of the methods, shown by running the EMIT calibrators in the radioimmunoassay method and vice versa. The choice of thyroxine method in the individual laboratory will then depend on considerations other than methodological factors.