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. 2020 Mar;34(2):607-615.
doi: 10.1111/jvim.15703. Epub 2020 Feb 3.

Comparison of 2 assays for measuring serum total thyroxine concentration in dogs and cats

Ewan D S Wolff  1 , Graham Bilbrough  2 , George Moore  1 , Lynn Guptill  1 , J Catharine Scott-Moncrieff  1
Affiliations

Comparison of 2 assays for measuring serum total thyroxine concentration in dogs and cats

Ewan D S Wolff et al. J Vet Intern Med. 2020 Mar.

Abstract

Background: No gold standard assay for serum total thyroxine (TT4) concentration in small animals exists. The Microgenics DRI TT4 (MTT4) assay is used by most reference laboratories.

Hypothesis/objectives: IDEXX Catalyst Total T4 (CTT4) and Immulite 2000 TT4 (ITT4) results will agree with MTT4 results.

Animals: Residual small animal sera were randomized before reanalysis (dogs, CTT4 versus MTT4: n = 176, ITT4 versus MTT4: n = 74; cats, CTT4 versus MTT4: n = 319, ITT4 versus MTT4: n = 79).

Methods: Validation and method comparison study. Serum TT4 concentration was measured on all analyzers. Pairwise Pearson correlation, cumulative sum linearity test, regression, and Bland-Altman method were performed.

Results: CTT4 versus MTT4 in dogs: constant bias (y-intercept) was 0.10 μg/dL (95% confidence interval [CI], 0.05-0.15), proportional bias (slope) was 0.86 μg/dL (95% CI, 0.83-0.89); in cats, constant bias was 0.13 μg/dL (95% CI, 0.08-0.20) and proportional bias was 1.01 μg/dL (95% CI, 0.98-1.03), but the test for linearity failed. Bland-Altman plots identified increasing disagreement with increasing serum TT4 concentrations. ITT4 versus MTT4 in dogs, constant bias was 0.14 μg/dL (95% CI, 0.04-0.22) and 0.22 μg/dL (95% CI, 0.09-0.33) for cats; proportional bias was 0.76 (95% CI, 0.72-0.80) for dogs and 0.71 (95% CI, 0.69-0.74) for cats.

Conclusions and clinical importance: Differences in CTT4 and MTT4 results affect interpretation at higher serum TT4 concentrations. The ITT4 proportional bias will underestimate serum TT4 concentrations in dogs and cats, compared to MTT4. Serial TT4 measurements should be done using the same assay.

Keywords: chemiluminescence; enzyme immunoassay; hyperthyroidism; hypothyroidism.

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Conflict of interest statement

Dr. Moore serves as Consulting Editor for Experimental Design and Statistics for the Journal of Veterinary Internal Medicine. He was not involved in review of this manuscript. Dr. Scott‐Moncrieff receives honoraria for speaking for IDEXX Laboratories. Dr. Wolff's expenses for carrying out this study were supported by IDEXX. IDEXX also paid for his travel to present an abstract of these findings at ECVIM‐CA. Dr. Bilbrough is employed by IDEXX and has received gifts from and stock options in the company. Because of this potential conflict all final decisions about study design and study reporting were the responsibilities of the other authors. The study design in which samples were divided into aliquots and coded by another author (Wolff) blinded IDEXX to sample identity. Dr. Bilbrough was not involved in preparation of the manuscript other than to review it before submission.

Figures

Figure 1
Figure 1
A, Passing‐Bablok regression of canine CTT4 versus MTT4. The regression equation for the Passing‐Bablok regression was y = 0.103 + 0.860x. The solid line represents the data regression line with dashed lines representing the confidence intervals. B, Bland‐Altman plot of CTT4 versus MTT4 for dogs. The solid line represents the mean difference (−0.28 μg/dL [−3.60 nmol/L]) and the dashed lines represent the mean difference ±1.96 SD (−1.43‐0.86 μg/dL [−18.41‐11.07 nmol/L]). CTT4, Catalyst Total T4 Test; MTT4, Microgenics DRI human TT4 EIA assay
Figure 2
Figure 2
A, Passing‐Bablok regression of canine ITT4 versus MTT4. The regression equation for the Passing‐Bablok regression was y = 0.144 + 0.762x. The solid line represents the data regression line with dashed lines representing the confidence intervals. B, Bland‐Altman plot of ITT4 versus MTT4 for dogs. The solid line represents the mean difference (−0.66 μg/dL [8.50 nmol/L]) and the dashed lines represent the mean difference ±1.96 SD (−2.08‐0.75 μg/dL [−26.77‐9.65 nmol/L]). ITT4, Immulite 1000 chemiluminescent TT4 assay; MTT4, Microgenics DRI human TT4 EIA assay
Figure 3
Figure 3
A, Passing‐Bablok regression line of feline CTT4 versus MTT4 is shown but the assumption of linearity was violated. Piecewise regression analysis yielded a slope of 1.25 (95% CI, 0.93‐1.56) indicating a tendency to overestimate TT4 concentration until the MTT4 breakpoint of 1.80 μg/dL (95% CI, 0.94‐2.66 μg/dL) [23.17 nmol/L; 95% CI, 12.10‐34.24 nmol/L). The second linear segment from 1.80 μg/dL [23.17 nmol/L) had a slope of 0.94 (95% CI, 0.93‐0.96), indicating proportional bias with subsequent underestimation of TT4 concentrations. B, Bland‐Altman plot of CTT4 versus MTT4 for cats. The solid line represents the mean difference (0.10 μg/dL [1.29 nmol/L]) and the dashed lines represent the mean difference ±1.96 SD (−1.19‐1.30 μg/dL [−15.32‐16.73 nmol/L]). CI, confidence interval; CTT4, Catalyst Total T4 Test; MTT4, Microgenics DRI human TT4 EIA assay
Figure 4
Figure 4
A, Passing‐Bablok regression of feline ITT4 versus MTT4. The regression equation for the Passing‐Bablok regression was y = 0.220 + 0.710x. The solid line represents the data regression line. B, Bland‐Altman plot of ITT4 versus MTT4 for cats. The solid line represents the mean difference (−1.11 μg/dL [−14.29 nmol/L]) and the dashed lines represent the mean difference ±1.96 SD (−2.58‐0.35 μg/dL [−33.21‐4.51 nmol/L]). ITT4, Immulite 1000 chemiluminescent TT4 assay; MTT4, Microgenics DRI human TT4 EIA assay
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