Four nonisotopic immunoassays of free thyroxin evaluated.

We evaluated four new nonisotopic immunoassays of free thyroxin (FT4)--Amerlite FT4 (Amersham International), Magic Lite FT4 (Ciba Corning Diagnostics), Stratus FT4 (Dade--Baxter Travenol), and FT4 Enzelsa (Compagnie ORIS Industrie)--by comparison with two FT4 radioimmunoassays: Amerlex and Sclavo. Inter- and intra-assay coefficients of variation were less than 10% in the working range and there was no significant interaction between the tracer conjugate and albumin. All methods except Enzelsa gave results equivalent to those obtained with the Sclavo chromatographic technique. In a population consisting of 325 euthyroid subjects and 111 hyperthyroid and 61 hypothyroid patients without any treatment, we observed slight overlaps between the hyperthyroid and euthyroid populations and diagnostic sensitivities were approximately 0.95 and 0.80 for hyperthyroidism and hypothyroidism. In specific populations (69 patients with chronic renal failure, 20 patients in intensive-care, 17 patients treated with heparin, and 27 pregnant women), results were quantitatively similar to those obtained by the Sclavo assay. We conclude that these nonisotopic methods are a valid alternative to current radioisotopic commercial methods.