Biological variation of total thyroxine (T4), free T4 and thyroid-stimulating hormone in 11 clinically healthy cats

Evaluation of the analytical performance of a point-of-care analyzer for the measurement of feline serum thyroxine concentration in comparison with a chemiluminescence enzyme immunoassay

BACKGROUND

Total thyroxine (TT4) measurement is used to assess thyroid status in cats.

OBJECTIVES

The aim of the prospective study was to evaluate the analytical performance of the point-of-care analyzer (POCA) Immuno AU10V using the v-T4 test kit for feline TT4 measurement. Additionally, method comparison with a benchtop analyzer (IMMULITE 2000) was done.

METHODS

Validation included linearity, inter- and intra-assay precision, precision near the lower limit of quantification (LloQ), and interference testing for hemoglobin, lipid, and bilirubin. Correlation and bias were assessed.

RESULTS

Linearity was given within the dynamic range. Coefficients of variation (CV) were ≤4% near the LloQ as well as for intra-and inter-assay precision. No interference was observed for lipid and bilirubin, while hemoglobin caused a negative bias of 28%. Method comparison included 74 samples within three TT4 concentration ranges (0.5-3.7, >3.7-5.13, >5.13-8 μg/dL). Correlation between POCA and reference method was excellent (rs = 0.95) with a slight proportional bias of 4.5%. TEobs was between 7.0% and 9.8%. Despite substantial agreement, discordant results on thyroid status occurred in 15% of samples.

CONCLUSIONS

The analytical performance of the POCA was excellent, as was its correlation with the reference method. Except for the interferent effect of hemoglobin, the TEobs was 8 μg/dL, and de novo reference intervals need to be established.

Clinical importance of borderline proteinuria in nonazotemic cats and evaluation of other risk factors for the development of chronic kidney disease

BACKGROUND

Borderline proteinuria is associated with decreased survival in cats with azotemic chronic kidney disease (CKD).

OBJECTIVES

Determine the clinical importance of borderline proteinuria in nonazotemic cats.

ANIMALS

A total of 201 healthy client-owned cats ≥7 years of age; 150 nonproteinuric (urinary protein : creatinine ratio [UPC] <0.2) and 51 borderline proteinuric (UPC 0.2-0.4).

METHODS

Prospective study. Cats were thoroughly screened and subsequently examined every 6 months for 2 years. Kaplan-Meier curves were compared between nonproteinuric and borderline proteinuric cats. Univariable and multivariable Cox models were fit to determine the relationship between development of renal disease and potential risk factors such as age, sex, breed, weight, dental disease, blood pressure, serum creatinine concentration (sCrea), serum symmetric dimethylarginine concentration (sSDMA), blood urea nitrogen concentration, urine specific gravity (USG), and UPC.

RESULTS

Significantly more cats with borderline proteinuria at inclusion developed renal disease (International Renal Interest Society [IRIS] ≥ stage 2 CKD or renal proteinuria; log-rank P = .004) or died (log-rank P = .02) within 2 years, compared with nonproteinuric cats. In the multivariate analysis, IRIS stage 1 CKD (persistent USG <1.035 or sSDMA >14 μg/dL; hazard ratio [HR], 4.2; 95% confidence interval [CI], 2.0-8.8; P < .001), sCrea ≥1.6 mg/dL (≥140 μmol/L; HR, 2.6; 95% CI, 1.1-6.4; P = .04), borderline proteinuria (HR, 2.5; 95% CI, 1.2-5.2; P = .01), and age at inclusion (HR, 1.3; 95% CI, 1.2-1.5; P < .001) were significantly associated with diagnosis of renal disease 6 months later.

CONCLUSIONS AND CLINICAL IMPORTANCE

Borderline proteinuria should receive more attention in healthy mature adult and senior cats because it is associated with renal disease and death.

Value of repeated health screening in 259 apparently healthy mature adult and senior cats followed for 2 years

BACKGROUND

Although regular health screening is recommended, long-term follow-up data in healthy aged cats are lacking.

OBJECTIVES

Determine the most common conditions in a large group of apparently healthy older cats and which diseases are manifested within 2 years in cats confirmed to be healthy based on extensive health screening.

ANIMALS

Client-owned cats.

METHODS

Prospective study. Thorough history, physical examination, blood tests, and urinalysis were performed in 259 apparently healthy mature adult (7-10 years) and senior (>10 years) cats. Semi-annual follow-up examinations were performed in 201 confirmed healthy cats.

RESULTS

At baseline, 21% of apparently healthy cats were not considered healthy but were diagnosed with International Renal Interest Society (IRIS) ≥ stage 2 chronic kidney disease (CKD; 7.7%) or hyperthyroidism (4.6%), among other disorders. Disease occurred significantly more frequently in senior cats compared with mature adult cats. In addition, 40% cats were overweight, 35% had moderate to severe dental disease, and 22% had abnormal cardiac auscultation findings. Within 2 years, 28% of mature adult and 54% of senior cats that were confirmed healthy at inclusion developed new diseases, most commonly IRIS ≥ stage 2 CKD (cumulative incidence, 13.4%), hyperthyroidism (8.5%), chronic enteropathy, hepatopathy or pancreatitis (7.5%), or neoplasia (7%).

CONCLUSIONS AND CLINICAL IMPORTANCE

The high prevalence and 2-year incidence of physical examination abnormalities and systemic diseases in apparently healthy older cats argue for regular health screening in cats ≥7 years of age. Although more common in senior cats, occult disease also occurs in mature adult cats, and owners should be informed accordingly.

Biological variation of urinary protein: Creatinine ratio and urine specific gravity in cats

BACKGROUND

Laboratory results are influenced by presence and severity of disease, as well as preanalytical factors, analytical variation, and biological variation. Biological variation data for urinary protein: creatinine ratio (UPC) and urine specific gravity (USG) in cats are lacking.

OBJECTIVES

Determine the biological variation of UPC and USG in cats.

ANIMALS

Eighty healthy client-owned cats.

METHODS

Prospective study. Urine was collected on days 0, 14, and 56 from all 80 cats to investigate the persistence of borderline or overt proteinuria or suboptimal urine concentration. In 15 of these cats, urine was collected weekly from day 0 to 42 to calculate the index of individuality (II) and reference change value (RCV), and on days 56 and 57 to evaluate day-to-day variability of UPC and USG.

RESULTS

Borderline or overt proteinuria (UPC ≥0.2) was present in 18/80 (23%) cats at baseline and persisted on 3 occasions in 2 months in 8/18 (44%) cats. Urine concentration was suboptimal at inclusion (USG <1.035) in 8/80 (10%) cats and at all 3 time points during 2 months in 3/8 (38%) cats. The II of UPC and USG indicated intermediate individuality. The 1-sided RCV was 82% for UPC and 36% for USG. Proteinuria substage was identical on 2 consecutive days in 13/15 (87%) cats, and urine concentrating ability remained the same in all 15 cats.

CONCLUSIONS AND CLINICAL IMPORTANCE

A >82% increase in UPC in a healthy cat is not solely attributable to physiological and analytical variation. For USG, a decrease of >36% is considered clinically relevant.

Biological variation of serum thyrotropin and thyroid hormones concentrations determined at 8-week intervals for 1 year in clinically healthy cats

BACKGROUND

Cats commonly develop thyroid disease but little is known about the long-term biological variability of serum thyroid hormone and thyrotropin (thyroid-stimulating hormone; TSH) concentrations.

OBJECTIVES

We aimed to determine the long-term biological variation of thyroid hormones and TSH in clinically healthy cats.

METHODS

A prospective, observational study was carried out. Serum samples for analysis of total thyroxine (T4, by radioimmunoassay [RIA] and homogenous enzyme immunoassay [EIA]), triiodothyronine (T3 ), free T4 (by dialysis), and TSH were obtained every 8 weeks for 1 year from 15 healthy cats, then frozen until single-batch analysis. Coefficients of variation (CV) within individual cats (CV I ) and among individual cats (CV G ), as well as the variation between duplicates (ie, analytical variation [CV A ]) were determined with restricted maximum likelihood estimation. The indices of individuality (IoI) and reference change values (RCVs) for each hormone were calculated.

RESULTS

Some thyroid hormones showed similar (total T4 by EIA) or greater (TSH) interindividual relative to intraindividual variation resulting in intermediate to high IoI, consistent with previous studies evaluating the biological variation of these hormones weekly for 5-6 weeks. By contrast, total T4 (by RIA) and free T4 had a low IoI. Total T3 had a high ratio ofCV A toCV I ; therefore, interindividual variation could not be distinguished from analytical variation. No seasonal variability in the hormones could be demonstrated.

CONCLUSIONS

Clinicians might improve the diagnosis of feline thyroid disease by establishing baseline concentrations for analytes with intermediate-high IoI (total T4, TSH) for individual cats and applying RCVs to subsequent measurements.

Biological variation of biochemical analytes determined at 8-week intervals for 1 year in clinically healthy cats

BACKGROUND

Biological variation helps determine whether population-based or subject-based reference intervals are more appropriate to assess changes in serial analytical values. Previous studies have investigated the biological variation of biochemical analytes weekly or with variable frequency over 5-14 weeks in cats, but none have considered biological variation at less frequent intervals over 1 year.

OBJECTIVES

We aimed to evaluate the long-term biological variation of 19 biochemical analytes in clinically healthy cats.

METHODS

A prospective, observational study in which 15 clinically healthy, client-owned cats were sampled for serum biochemical analyses every 8 weeks for 1 year. Frozen serum samples were single-batch analyzed. Restricted maximum likelihood estimation was used to determine the coefficients of variation (CV), describing variation within each cat, between cats, and the analytical variation. These CVs were used to determine the indices of individuality and reference change values (RCVs).

RESULTS

Albumin, alkaline phosphatase, creatine kinase, and globulin had high indices of individuality, indicating that they are best evaluated by RCVs. Phosphorus, potassium, chloride, sodium, symmetric dimethylarginine, and total CO₂ had low indices of individuality, indicating that population-based reference intervals are appropriate. Alanine aminotransferase, aspartate aminotransferase, blood urea nitrogen, calcium, cholesterol, creatinine, glucose, total bilirubin, and total protein had intermediate indices of individuality, indicating that RCVs may provide additional insight into the interpretation of analyte measurements beyond the population-based reference intervals.

CONCLUSIONS

For many analytes, the biological variation detected was similar to that reported in prior studies. Clinicians should consider the biological variation of analytes to best interpret clinically relevant changes in serial analyte measurements.