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Analytic performance evaluation of a veterinary-specific ELISA for measurement of serum cortisol concentrations of dogs

Michael B. LaneDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

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Bente FlatlandDepartment of Biomedical and Diagnostic Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

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Shelly J. OlinDepartment of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

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Kellie A. FecteauDepartment of Biomedical and Diagnostic Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

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Markus RickDepartment of Pathobiology and Diagnostic Investigation, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824.

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Luca GioriDepartment of Biomedical and Diagnostic Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.

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Abstract

OBJECTIVE To investigate the precision of an ELISA for measurement of serum cortisol concentration (SCC) in dogs, assess agreement between this ELISA and 2 validated chemiluminescence assays (CLAs), and evaluate the clinical implications of any bias associated with this ELISA when measuring SCC in dogs.

DESIGN Evaluation study.

SAMPLE 75 stored, frozen serum samples from client-owned dogs.

PROCEDURES Enzyme-linked immunosorbent assay precision was evaluated by measuring SCC of pooled serum samples. Agreement with standard methods was evaluated with Spearman rank correlation, Passing-Bablok regression, and Bland-Altman analysis to compare SCCs obtained with the ELISA and the 2 CLAs. An error grid was used to evaluate identified bias.

RESULTS Within-laboratory coefficients of variation for pooled serum samples with low, medium, and high SCCs were 21.4%, 28.9%, and 13.0%, respectively. There was a high correlation between ELISA results (for all samples combined) and results of the 2 CLAs (CLA 1, r = 0.96; CLA 2, r = 0.97), but constant and proportional biases between the ELISA and CLAs were present at all concentrations. Clinically important disagreement between ELISA results and CLA results occurred in 16 of 63 (25%) samples, particularly with low and high SCCs.

CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that the rate of clinical disagreement between the ELISA and CLAs was sufficiently high to recommend that equivocal results obtained with the ELISA be confirmed by a reference laboratory. Further evaluation of analytic performance of the ELISA should focus on samples with very high and very low SCCs.

Abstract

OBJECTIVE To investigate the precision of an ELISA for measurement of serum cortisol concentration (SCC) in dogs, assess agreement between this ELISA and 2 validated chemiluminescence assays (CLAs), and evaluate the clinical implications of any bias associated with this ELISA when measuring SCC in dogs.

DESIGN Evaluation study.

SAMPLE 75 stored, frozen serum samples from client-owned dogs.

PROCEDURES Enzyme-linked immunosorbent assay precision was evaluated by measuring SCC of pooled serum samples. Agreement with standard methods was evaluated with Spearman rank correlation, Passing-Bablok regression, and Bland-Altman analysis to compare SCCs obtained with the ELISA and the 2 CLAs. An error grid was used to evaluate identified bias.

RESULTS Within-laboratory coefficients of variation for pooled serum samples with low, medium, and high SCCs were 21.4%, 28.9%, and 13.0%, respectively. There was a high correlation between ELISA results (for all samples combined) and results of the 2 CLAs (CLA 1, r = 0.96; CLA 2, r = 0.97), but constant and proportional biases between the ELISA and CLAs were present at all concentrations. Clinically important disagreement between ELISA results and CLA results occurred in 16 of 63 (25%) samples, particularly with low and high SCCs.

CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that the rate of clinical disagreement between the ELISA and CLAs was sufficiently high to recommend that equivocal results obtained with the ELISA be confirmed by a reference laboratory. Further evaluation of analytic performance of the ELISA should focus on samples with very high and very low SCCs.

Contributor Notes

Address correspondence to Dr. Giori (lgiori@utk.edu).