Evaluation of the manual enzyme immunoassay (EMIT) procedure for determination of serum thyroxine.

Our experience with the determination of thyroxine (T4) in serum using a homogeneous enzyme immunoassay technique (EMIT, Syva Corp.) is reported. The intra-assay precision of the EMIT Manual Thyroxine Assay was investigated with 2 different calibrator concentrations and showed coefficients of variation varying from 5--2% for thyroxine concentrations of 40 and 120 micrograms/l thyroxine respectively. The inter-assay precision was investigated with different series of calibrators and serum specimens. Coefficients of variation for the calibrators varied from 35--5% in the range of 20--200 micrograms/l thyroxine and for the serum specimens in the range of 8--232 micrograms/l thyroxine from 50--4%. The recovery of various amounts of thyroxine added to thyroxine-free serum varied between 91--103%. The cross reactivity of structurally related compounds such as: monoiodothyronine, diiodothyronine, triiodothyronine, triiodothyroacetic acid and tetraiodothyroacetic acid was investigated. Serum samples of 100 patients were analysed by EMIT and radioimmunoassay (T4 RIA (PEG), Abbott Lab.). A good correlation was found between the EMIT and RIA assay (r = 0.96, slope = 0.96 and y-intercept = 3.37 micrograms/l).