Results of the world wide clinical trial of the Amerlex free T4 RIA kit

The direct assay of serum free T4 by traditional means, namely equilibrium dialysis and ultrafiltration, has been available for many years but has not been ideally suited to routine use on a wide scale. A novel approache to the measurement of free T4 in serum has been developed that provides a direct single tube RIA method which is convenient for routine thyroid screening. The principle of this, the Amerlex free T4 assay, is based on the use of a 125I-T4 derivative that has been modified to severely inhibit its binding to the major binding proteins in serum, whilst retaining its normal binding to a high avidity specific T4 antiserum. In use, the assay resembles a conventional solid phase RIA. The free T4 and 125I-T4 derivative compete for binding to a limited quantity of antibody, the assay reaching equilibrium in about 1 hour. Bound tracer is separated by centrifugation and the free T4 concentration is interpolated from the count rate with reference to serum calibrators. This free T4 assay has been subjected to an extensive world wide clinical trial and a summary of the results from 39 centres in 14 countries is presented. The precision of the assay has been shown to be excellent in the important clinical regions and the assay correlates well with the reference method of equilibrium dialysis. The clinical discrimination has been shown to be generally good, especially at the euthyroid-hyperthyroid borderline. In conclusion, this extensive trial on over 5800 patients has demonstrated the applicability of this new and convenient free T4 assay for the initial thyroid function screening of a wide variety of patients