Assessment of serum thyroxine binding capacity-dependent biases in free thyroxine assays

Background: Free thyroxine (FT4) assays may exhibit biases that are related to serum T-4 binding capacity (sBC. We describe two tests that can be used to assess the presence and magnitude of sBC-dependent biases in FT4 assays . Methods: We used a direct-equilibrium dialysis FT4 assay as the reference m ethod and compared the results obtained with those of the FT4 assays under investigation, in patient sera having a wide range of sBC, We then compared the expected and observed FT4 results for sera diluted with an inert buffe r. Because serum dilution causes a predictable decrease in sBC, an increasi ngly negative,bias on progressive dilution is indicative of a sBC-dependent bias. Results: The automated FT4 assay investigated. (Vitros FT4) showed no demon strable sBC-dependent bias by either test. Conclusion: These two tests can be used to screen for sBC-dependent biases in FT4 assays. (C) 1999 American Association for Clinical Chemistry.