Diagnostic cut-off values for the urinary corticoid:creatinine ratio for the diagnosis of canine Cushing's syndrome using an automated chemiluminescent assay.

Background: Cushing's syndrome is one of the most common endocrinopathies in dogs. The preferred screening test for spontaneous Cushing's syndrome is the low-dose dexamethasone suppression test (LDDST). The diagnostic value of urinary cortisol:creatinine ratios (UCCR) is questionable.

Objectives: The aim of this study was to determine diagnostic cut-off values for UCCR testing in comparison with LDDST as a clinical reference standard and to calculate the sensitivity and specificity.

Methods: Data from 2018 to 2020 were obtained retrospectively from a commercial laboratory. Both LDDST and UCCR were measured by automated chemiluminescent immunoassay (CLIA). The maximum interval between both tests was 14 days. The optimal cut-off value for UCCR testing was calculated by the Youden index. The sensitivity and specificity of these cut-off values for the UCCR test and LDDST were assessed by Bayesian latent class models (BLCMs).

Results: This study included 324 dogs with both UCCR test and LDDST results. The optimal UCCR cut-off value, calculated by the Youden index, was 47.4 × 10 . Any UCCR <40 × 10 was interpreted as a negative result, 40-60 × 10 as values in a gray zone, and >60 × 10 as positive. Using the cut-off of 60 × 10 , BLCM showed 91% (LDDST) and 86% (UCCR test) sensitivity and a specificity of 54% (LDDST) and 63% (UCCR test).

Conclusions: Considering an 86% sensitivity and a 63% specificity, UCCR testing may be considered a first-line investigation to rule out Cushing's syndrome using CLIA analysis. Urine samples can be collected noninvasively at home by the owner, reducing the potential impact of stress.