Evaluation of an automated, homogeneous enzyme immunoassay for serum thyroxine measurement in dog and cat serum.

A homogenous enzyme immunoassay (EIA) for measurement of serum thyroxine (T4) concentration was evaluated for use with canine and feline serum. The EIA method was linear from 0 to 150 nmol T4/L for human serum, 0 to 94 nmol T4/L for feline serum and 10 to 60 nmol T4/L for canine serum. Intra- and interassay precision studies yielded coefficients of variation </= 8% using single point measurements. Method comparison studies gave close agreement between radioimmunoassay (RIA) and EIA results. Correlation coefficients (r values) were 0.88 and 0.97 for canine and feline samples respectively, after application of species-specific factors to correct the calibrator values assigned for human serum samples. The EIA showed no interference from hemolysis at hemoglobin concentrations </= 20 g/L or from moderate lipemia. Highly lipemic specimens could be tested after centrifugation to clarify the sample. The EIA showed less interference from autoantibodies to T4 than the RIA method. The EIA method allows automation of T4 testing in a veterinary hospital or laboratory, and can be integrated with the routine clinical chemistry panel.