Evaluation of an enzyme linked immunosorbent assay for the determination of free thyroxine in canine plasma samples assisted by data on biological variation.

The purpose of the present study was to evaluate a commercially available enzyme linked immunosorbent assay (ELISA) for the determination of the free thyroxine (FT4) concentration in canine plasma samples. The test kit Enzymun-Test (Boehringer Mannheim) for determination of FT4 concentrations in human serum and plasma was used in connection with the analyzer Enzymun-Test(Boehringer Mannheim) System ES-33 by following the assay protocol suggested by the manufacturer. To assist in the evaluation of the ELISA method, goals for the precision of the assay were established from the inter- and intraindividual coefficients of variation estimated from determination of the plasma FT4 concentration in blood samples collected weekly from 8 healthy dogs for a period of 4 weeks. The intra- and interassay coefficients of variation ranged from 6% to 13%, the proposed goals for the precision ranging between 10% and 15%. The detection limit of the assay was 2 pmol/l which is well below the FT4 concentration commonly observed in plasma from dogs, and the FT4 concentrations determined in plasma from healthy dogs were comparable to those reported in the literature. Hence, the ELISA method evaluated in this study offers a precise and accurate measurement of the FT4 concentration in canine plasma samples, the inter- and intraassay precision comparing favorably to the proposed specifications for assay precision using estimates of the inter- and intraindividual coefficients of variation.