A comparative study of free thyroxine estimations

Two commercial methods (Immophase, Corning and FT4, Lepetit) for the measurement of free thyroxine (FT4) in serum have been experimentally evaluated. Ease of handling and rapidity were satisfactory. Precision profiles showed intra-assay coefficients of variation not exceeding 5% over the entire measurement range (0-70 pmol/l) for both methods. Interassay coefficients of variation were below 8%. Assay results of both methods correlated well with those obtained with a symmetric dialysis method, with correlation coefficients of 0.95 for the Immophase and 0.93 for the Lepetit method and slopes not significantly different from unity. Comparison with this reference method did not give any indication of serious methodological error in either of the kits in spite of theoretical doubts with regard to one of them.