Immunoassay antibody modification significantly impacts low-dose dexamethasone suppression test cutoffs in dogs with...

To reassess the 8-hour postdexamethasone (T8) cortisol diagnostic cutoff for suspected naturally occurring Cushing syndrome (CS) in dogs comparing a chemiluminescent cortisol immunoassay (Immulite 2000 XPi; Siemens Healthineers) results before and after the 2020 antibody change.

Low-dose dexamethasone suppression test results were retrospectively evaluated in dogs with CS and in those with disease mimicking Cushing syndrome (DMCS), with sensitivity (Se), specificity (Sp), and receiver operating characteristic curves assessed. Cortisol concentrations after the antibody change were adjusted using the Siemens Healthineers conversion formula.

61 dogs with CS and 32 with DMCS were included between January 2016 and October 2020 (old antibody [OA]). Forty dogs with CS and 40 with DMCS were included between November 2020 and January 2024 (new antibody [NA]). The area under the curve for T8 cortisol to differentiate CS from DMCS dogs was 0.96 (95% CI, 0.91 to 0.99) with OA and 0.96 (95% CI, 0.90 to 1.0) with NA. The cutoff associated with the best sensitivity and specificity to diagnose CS was > 35.9 nmol/L (1.3 μg/dL; Se = 88.9%, 95% CI, 78.4% to 95.4%; Sp = 96.9%, 95% CI, 83.8% to 99.9%) with OA and > 33.1 (1.2 μg/dL; Se = 92.5%, 95% CI, 79.6% to 98.4%; Sp = 95.0%, 95% CI, 83.08% to 99.4%) with NA.

The optimal cut point of the low-dose dexamethasone suppression test T8 cortisol for CS diagnosis after the antibody change was > 33.1 nmol/L (1.2 μg/dL), which is lower than the currently accepted cutoff of > 38.6 nmol/L (1.4 μg/dL).

New cutoff value of T8 cortisol has been identified for the diagnosis of CS.