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

Update LDDST T8 cortisol cutoff to >33.1 nmol/L when using the post-2020 Immulite 2000 XPi antibody formulation.

This retrospective study investigated the impact of a 2020 antibody modification in the Immulite 2000 XPi chemiluminescent cortisol immunoassay (Siemens Healthineers) on the diagnostic cutoff for the 8-hour post-dexamethasone (T8) cortisol concentration in the low-dose dexamethasone suppression test (LDDST) for canine Cushing syndrome (CS). The study compared LDDST results from 93 dogs (61 CS, 32 disease mimicking Cushing syndrome [DMCS]) tested with the old antibody (OA; January 2016–October 2020) versus 80 dogs (40 CS, 40 DMCS) tested with the new antibody (NA; November 2020–January 2024). Receiver operating characteristic analysis demonstrated excellent discriminatory ability in both groups, with an area under the curve of 0.96 for both OA and NA cohorts. The optimal T8 cortisol cutoff for CS diagnosis was >35.9 nmol/L (1.3 μg/dL) with OA (sensitivity 88.9%, specificity 96.9%) and >33.1 nmol/L (1.2 μg/dL) with NA (sensitivity 92.5%, specificity 95.0%). Importantly, the currently accepted diagnostic cutoff of >38.6 nmol/L (1.4 μg/dL) is no longer appropriate following the antibody reformulation, as it would reduce diagnostic sensitivity. The authors also note that Siemens' provided conversion formula does not adequately correct for this analytical shift. Clinicians using the Immulite 2000 XPi platform after November 2020 should apply the updated T8 cortisol cutoff of >33.1 nmol/L to optimize CS diagnosis and minimize missed diagnoses.

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