Serum thyroxine binding capacity-dependent bias in an automated free thyroxine assay

The magnitude of serum thyroxine (T4) binding capacity (sBC) dependent bias in the AXSYM free thyroxine (FT4) assay was assessed using two recently de scribed tests. One of the tests uses a direct equilibrium dialysis (ED) FT4 assay as the reference method. The results obtained with the AXSYM method were compared with those obtained by the ED FT4 method in patient sera havi ng a wide range of sBC The other test involves comparison of the FT4 result s obtained following dilution of sera by an inert buffer, to theoretically derived FT4 results. As serum dilution causes a predictable decrease in sBC , the demonstration of a negative bias whose magnitude increases in paralle l to the dilution, is indicative of an sBC-dependent bias. The AXSYM FT4 as say exhibited a significant sBC-dependent bias. This sBC-dependent bias is likely to have been caused by the presence of significant amounts of T4 bin ding proteins in the assay reagents.