Comparison of in-clinic point-of-care and reference laboratory total thyroxine immunoassays for diagnosis and post-treatment monitoring of hyperthyroid cats

Abstract

Objectives The Catalyst One Chemistry Analyzer (IDEXX Laboratories) is a point-of-care instrument that can measure total thyroxine (TT4) by immunoassay. The aims of this study were to evaluate the analytic performance of the Catalyst TT4 assay in feline sera and to examine agreement of the Catalyst TT4 results with those measured by immunoassay at a veterinary reference laboratory. Methods Assay precision, reproducibility and linearity were evaluated for the Catalyst TT4 assay. For method comparison, TT4 concentrations in serum samples from 157 cats (127 hyperthyroid, 30 radioiodine-treated cats) were analyzed by both in-clinic and reference laboratory methods. Results The Catalyst TT4 demonstrated good precision and reproducibility (coefficients of variation ⩽8.5%) and excellent linearity in the diagnostic range of 6-150 nmol/l. Differences between the two TT4 methods showed no proportional or fixed bias (Bland-Altman plots) but did demonstrate greater spread of values at higher TT4 concentrations. Statistical analysis of percent differences between methods indicated 95% limits of agreement of ± 30%. When serum TT4 concentrations were classified as low, high or within the reference interval (12-50 nmol/l) for each assay, there was strong agreement (96.8%) in classification between methods. Conclusions and relevance The Catalyst TT4 assay provided precise serum TT4 concentrations in the 157 samples analyzed, which agreed well with results provided by a reference laboratory. Cats with Catalyst TT4 concentrations near decision thresholds (eg, normal vs high) should either have TT4 concentration repeated a few weeks later and/or undergo further testing (eg, free T4, serum thyroid-stimulating hormone, thyroid scintigraphy) to determine thyroid status.

Figure 1

Linearity of the Catalyst Total T4 assay method for serum total thyroxine (TT4) concentration using pooled feline serum containing low to high TT4 concentrations. The line of equality is shown (dashed line), as well as the line of best fit (solid line). Each marker represents the mean of six measurements (two Catalyst One Chemistry Analyzers; three measurements each)

Figure 2

Limits of agreement plots for total thyroxine (TT4) concentrations measured by the in-clinic Catalyst One Chemistry Analyzer and the veterinary reference laboratory. (a) Actual data; (b) normalized differences (difference divided by average values); and (c) actual data for cats within or near the reference interval. In all plots, the thick dashed line represents the line of perfect agreement, the thin dashed lines represent the 95% limits of agreement and the thin solid line represents the regression line