Analytical and clinical agreement between the Siemens Immulite 2000 veterinary cortisol and standard cortisol assays in canine serum.

Both Siemens Immulite 2000 cortisol assays show excellent agreement and can be used interchangeably in canine patients.

This study evaluated the analytical and clinical agreement between the Siemens Immulite 2000 standard cortisol (SC) and veterinary cortisol (VC) assays in canine serum, following a 2020 antibody reformulation that introduced an approximate 23% negative bias in the SC assay. Residual serum samples from 105 dogs (154 total measurements) submitted to a veterinary diagnostic laboratory were measured using both assay kits on the same chemiluminescent immunoassay analyzer. Median cortisol concentrations were 4.19 μg/dL (VC) and 4.67 μg/dL (SC). Statistical analysis using Passing-Bablok regression revealed no significant constant bias (intercept -0.035; 95% CI, -0.184 to 0.061) or proportional bias (slope 1.036; 95% CI, 0.997-1.098). Bland-Altman analysis confirmed no significant mean bias (-0.494 μg/dL), though limits of agreement were relatively wide (-4.353 to 5.342 μg/dL). Correlation between assays was excellent (Spearman's rho = 0.981). Diagnostic agreement was also excellent (Cohen's κ = 0.961), with only 3 of 105 cases classified discordantly, and these occurred near established diagnostic decision thresholds. The findings suggest that despite the antibody change that prompted development of the VC assay, both assays perform comparably in clinical practice. Clinicians and laboratory professionals can use either assay with confidence, though awareness of potential discordance near cutoff values remains important for borderline cases.

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