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

Radioiodine treatment of hyperthyroidism in cats: results of 165 cats treated by an individualised dosing algorithm in Spain

OBJECTIVES

Although radioiodine (131I) is the treatment of choice for feline hyperthyroidism, 131I-dosing protocols commonly induce iatrogenic hypothyroidism and expose azotaemia. A recently reported patient-specific 131I dosing algorithm minimised the risk of 131I-induced hypothyroidism and azotaemia, while maintaining high cure rates. The aim of the study was to report results of 131I treatment in a European population of hyperthyroid cats using this patient-specific dosing algorithm.

METHODS

This prospective case series (before-and-after study) evaluated 165 hyperthyroid cats referred for 131I treatment. All cats had serum concentrations of thyroxine (T4), triiodothyronine (T3) and thyroid-stimulating hormone (TSH) measured (off methimazole ⩾1 week). Thyroid volume and percentage uptake of 99mTc-pertechnetate (TcTU) were determined using thyroid scintigraphy. An initial 131I dose was calculated by averaging dose scores for T4/T3 concentrations, thyroid volume and TcTU; 70% of that composite dose was then administered. Twenty-four hours later, percentage 131I uptake was measured, and additional 131I administered as needed to deliver an adequate radiation dose to the thyroid tumour(s). Serum concentrations of T4, TSH and creatinine were determined 6-12 months later.

RESULTS

Median calculated 131I dose was 2.15 mCi (range 1.2-7.5), with only 51 (30.9%) receiving ⩾2.5 mCi. Of 165 cats, 124 (75.2%) became euthyroid, seven (4.2%) became overtly hypothyroid, 27 (16.4%) became subclinically hypothyroid and seven (4.2%) remained hyperthyroid. A higher proportion of overtly (85.7%) and subclinically (26.9%) hypothyroid cats developed azotaemia than euthyroid cats (13.6%; P = 0.0002). Hypothyroid cats were older (P = 0.016) and more likely to have detectable TSH concentrations (P = 0.025) and symmetrical bilateral distribution of 99mTc-pertechnetate uptake (P = 0.0002), whereas persistently hyperthyroid cats had higher severity scores (P = 0.012).

CONCLUSIONS AND RELEVANCE

Our results confirm that 131I dosing with this new algorithm results in high cure rates, with a lowered prevalence of 131I-induced overt hypothyroidism and azotaemia. Age, serum TSH concentrations, bilateral, symmetrical uptake and severity score help predict outcome.

BA-plotteR - A web tool for generating Bland-Altman plots and constructing limits of agreement

Investigators use Bland-Altman plot (Limits of Agreement plot) to compare two methods measuring the same continuous variable to determine interchangeability or agreement of the methods. The method has evolved to deal with heteroscedastic data and fixed or proportional biases (or both). Although an ordinary Bland-Altman plot can be readily made with various software applications, there is no free, open-source application that is dedicated to producing Bland-Altman plots and constructing limits of agreement for data that do not meet the assumptions of a simple comparison. To fill this gap, we created BA-plotteR, a web-based, open-source, freeware tool created in Shiny/R that is dedicated to creating Bland-Altman plots. We validated the tool using 20 datasets with various data distributions by comparing the output from the tool against manually derived results. The webtool handles data that requires a more complex analysis than is commonly available through commercial statistical programs. Moreover, the automated analysis of the data distribution will guide users and help them to correctly plot and analyse their data. The tool agreed perfectly with manually constructed plots. The Bland-Altman graphing tool provides clinical researchers with a tool that correctly analyzes and graphs studies involved in method comparisons. The tool can be accessed here: https://huygens.science.uva.nl/BA-plotteR.

New Tests in Feline Veterinary Medicine: When to Use Them and When to Stick with Tried-and-True Tests

The focus of this article is on how interpretations of laboratory data can utilize both population and individual reference intervals, while making the most of routine testing procedures coupled with some of the newer laboratory tests, which can complement the existing tests in diagnosing disease.

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

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.