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 T4 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 method 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 buffer. Because serum dilution causes a predictable decrease in sBC, an increasingly negative bias on progressive dilution is indicative of a sBC-dependent bias. RESULTS The automated FT4 assay investigated (Vitros FT4) showed no demonstrable sBC-dependent bias by either test. CONCLUSION These two tests can be used to screen for sBC-dependent biases in FT4 assays.