One-step chemiluminescent immunoassay of free thyroxin with acridinium-ester-labeled thyroxin evaluated and compared with a two-step radioimmunoassay.

This new one-step chemiluminescent immunoassay of free thyroxin (FT4) involving a thyroxin-immunoglobulin conjugate labeled with acridinium ester (Magic Lite System; Ciba Corning Diagnostics Corp., Medfield, MA) is rapid (one 1-h incubation), requires two calibrators per run, and takes 10 s per sample for the quantification step. Analytical performances were excellent: within- and between-run CVs of less than 10% in the working range, no significant effect of hemolysis, bilirubin, or lipemia, and no significant interaction between the conjugate and the thyroxin-binding proteins. Magic Lite results (y) correlated well with those obtained by the Sclavo (x) two-step radioimmunoassay (Sclavo, Siena, Italy): y = 1.35x + 1.32 (r = 0.94, n = 267, P less than 0.001, Sxy = 6.29). Clinical sensitivities (diagnostic efficiencies) for hypothyroidism and hyperthyroidism were 0.91 and 0.98 for normal interval limits of 12 and 21.5 pmol/L (95% confidence interval). Magic Lite results in situations where patient therapy, treatment, or unusual conditions can result in a lack of correlation between the clinical status and the FT4 values were qualitatively the same as those obtained by the Sclavo assay.