Counterpoint: Legitimate and Illegitimate Tests of Free-Analyte Assay Function: We Need to Identify the Factors That Influence Free-Analyte Assay Results

摘要

There have been, and continue to be, reports of discordant free thyroxine (T4)1 measurements when different free-T4 methods are applied to the same serum samples (1)(2)(3)(4). Our laboratory has attempted to identify some of the factors that contribute to these disparities.There are 4 separate components of serum T4: free T4, albumin-bound T4, transthyretin-bound T4, and thyroxine-binding globulin (TBG)-bound T4. Free T4 is the one component that moves across semipermeable membranes during equilibrium dialysis and ultrafiltration. As Midgley and Christofides point out, the gold standard free-T4 methods are designed to measure this component.We applied analog-type methods and an equilibrium-dialysis/immunoassay gold standard method to carefully prepared solutions in which we varied these 4 components of serum T4. This equilibrium-dialysis method agrees closely with a liquid chromatography–tandem mass spectrometry method applied to serum ultrafiltrates (4). We also applied this equilibrium dialysis/immunoassay and a group of 7 analog-type free-T4 assays to solutions of T4 that did not include T4-binding proteins (5); that is, the only form of T4 was free T4. The analog-type assays required 28- to 588-fold higher concentrations of free T4 than …