Pf. Solter et S. Farner, Correlation of two nonradioactive immunoassays to a radioimmunoassay technique for thyroxine measurement in equine serum, J VET D INV, 12(1), 2000, pp. 51-56
The purpose of this study was to compare 2 different nonradioactive assay m
ethods with a conventional radioimmunoassay (RIA) measuring the concentrati
on of serum thyroxine (T4) in horses. Serum was obtained from 85 adult stan
dardbred horses. The T4 concentration of each sample was analyzed by RIA, c
hemiluminescent enzyme immunoassay (CEI), and homogeneous enzyme immunoassa
y (HEI). The correlation between the HEI method and RIA method was signific
antly greater (r = 0.89) than the correlation between the CEI and the refer
ence method (r = 0.53). In addition, the precision of the HEI method was si
gnificantly greater than the CEI method; within-run percentage coefficients
of variation were 4.5% and 15.9%, respectively, at mean T4 concentrations
of 19-20 nmol/liter. On the basis of these findings, the HEI method was eva
luated further Both between-run precision and linearity were deemed adequat
e upon dilution by the HEI method. In addition, recovery of L-thyroxine add
ed to equine serum was linear over 6 concentrations tested and averaged 79.
6% with a manufacturer recommended data correction factor. An in-house corr
ection factor was calculated by linear regression analysis of the RIA and H
EI results from the original equine serum samples. Use of this correction f
actor improved the average recovery to 94.2% while maintaining excellent li
nearity (r(2) = 0.9978). Although both nonradioactive methods of T4 analysi
s could likely substitute for the RIA reference method, the HEI method had
the highest correlation and precision. The HEI technique also yielded adequ
ately accurate results after correction of the data with an appropriate cor
rection factor. Individually derived in-house correction factors may improv
e the accuracy of the HEI method to a greater extent than manufacturer sugg
ested correction factors.