SYSTEMATIC DIFFERENCES BETWEEN COMMERCIAL IMMUNOASSAYS FOR FREE-THYROXINE AND FREE TRIIODOTHYRONINE IN AN EXTERNAL QUALITY ASSESSMENT PROGRAM

Data collected in a national external quality assessment program for f ree thyroxine (fT(4)) and free triiodothyronine (fT(3)) were analyzed to evaluate the performance of 10 method/kits with 26 control samples distributed to similar to 170 laboratories. The control materials were normal serum pools, pooled sera supplemented with thyroid hormones, a pregnancy serum pool, serum pooled from patients with familial dysalb uminemic hyperthyroxinemia (FDH), and a normal serum pool progressivel y diluted. The between laboratory variability (CV) was approximately c onstant in normal and supplemented pools for fT(4) (15.3%) and fT(3) ( 24.0%) but markedly increased in diluted, pregnancy, and FDH pools (21 .9-35.2% for fT(4) and 28.6-66.5% for fT(3)) because of increases in s ystematic between-kit differences in control samples with altered bind ing-protein capacity. Moreover, free hormone concentrations measured i n progressively diluted sera averaged lower than in undiluted samples. This decrease of concentration was less for back-titration or labeled -antibody techniques and greater for labeled-analog methods; only the method involving adsorption to cross-linked dextran (Sephadex) was una ffected by dilution. Evaluation of the reproducibility of the method/k its showed between-assay, between-laboratory precision ranging from 7. 8% to 17.0% for fT(4) and from 9.8% to 20.3% for fT(3).