Evaluation of a point-of-care test for quantitative determination of total thyroxine in feline serum

Abstract

Objectives: Total thyroxine (TT4) evaluation is the most commonly used first-line test for the diagnosis and monitoring of cats with hyperthyroidism. Vcheck T4 is a point-of-care immunoassay that measures TT4 using a Vcheck V200 analyser. This study aimed to evaluate the analytic performance of the Vcheck T4 assay in feline sera and the agreement in the classification of normal, high and low TT4 concentrations of Vcheck T4 with those measured by an enzyme immunoassay (EIA).

Methods: Assay precision, reproducibility and linearity were evaluated for the Vcheck T4. For method comparison, TT4 concentrations in 73 serum samples were analysed by both methods.

Results: Vcheck T4 demonstrated good precision, reproducibility (intra- and inter-assay coefficients of variation between 3% and 13.5%) and linearity in the diagnostic range of <0.5 and >8 μg/dl. The correlation coefficient was 0.87, Passing-Bablok regression showed a proportional, but not constant bias, Bland-Altman plots revealed a mean difference of +0.5 μg/dl. The overall inter-rater agreement (K) between TT4 EIA and Vcheck results was substantial (K = 0.69), with 82.2% of concordant results. As a diagnostic test for hyperthyroidism, Vcheck T4 showed a sensitivity and specificity of 88.9% and 84.4%, and a positive predictive value (PPV) and negative predictive value (NPV) of 44.5% and 98.2%, respectively.

Conclusions and relevance: Vcheck T4 assay provided precise and reproducible results in substantial agreement with the EIA. Given the high NPV, Vcheck T4 is useful in ruling out hyperthyroidism when screening cats, but, based on low PPV, samples with results with high TT4 need to be analysed by another reference method. Further analysis with haemolytic, icteric and lipaemic samples is needed to assess the test performance. Finally, the reference intervals provided by the manufacturer require verification, and TT4 intervals specific to this method must be established in future studies.

Keywords: Total thyroxine; immunoassay; method comparison; point-of-care immunoassay.

Figure 1

Benchtop point-of-care analyser (Vcheck V200; Bionote) for Vcheck T4 dry slide analysis (on the left of the analyser)

Figure 2

Feline serum samples with normal (a) high (b) and low (c) total thyroxine (TT4) concentrations evaluated by enzyme immunoassay (EIA) and Vcheck T4 methods. Horizontal lines represent the interquartile range, red squares represent the median for each group and the shaded box represents the reference interval

Figure 3

Passing–Bablok regression of feline TT4 Vcheck T4 vs the enzyme immunoassay (EIA) method in 54 feline serum samples. The regression equation for the Passing–Bablok regression was y = 0.106 + 0.797x. The solid line represents the data regression line, with dashed lines representing the confidence intervals

Figure 4

Bland–Altman plot showing the difference between the measured feline serum total thyroxine (TT4) concentration by the Vcheck T4 and the enzyme immunoassay (EIA) method against the average TT4 measured by the Vcheck T4 and EIA in 54 feline serum samples. The solid line indicates the mean difference between the methods (+0.5 μg/dl) and the dashed lines represent limits of agreement, which are defined as the mean of the differences ±1.96 SD (lower limit, –1.40; upper limit, +2.50 μg/dl)