Immunoassay antibody modification significantly impacts low-dose dexamethasone suppression test cutoffs in dogs with Cushing syndrome
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Published on: July 22, 2011
Insights
A new T8 cortisol cutoff of > 33.1 nmol/L aids in diagnosing canine Cushing syndrome (CS). This updated value, following an immunoassay antibody change, improves diagnostic accuracy for CS in dogs.
Area of Science:
- Veterinary Medicine
- Endocrinology
- Diagnostic Assays
Background:
- Cushing syndrome (CS) is a common endocrine disorder in dogs.
- Accurate diagnosis relies on tests like the low-dose dexamethasone suppression test (LDDST).
- Assay methodology changes can impact diagnostic cutoffs.
Purpose of the Study:
- To re-evaluate the 8-hour postdexamethasone (T8) cortisol diagnostic cutoff for canine Cushing syndrome.
- To compare immunoassay results before and after a 2020 antibody change.
- To determine the optimal T8 cortisol cutoff for diagnosing naturally occurring CS in dogs.
Main Methods:
- Retrospective evaluation of LDDST results in dogs with confirmed CS and disease mimicking CS (DMCS).
- Assessment of sensitivity (Se) and specificity (Sp) using receiver operating characteristic curves.
- Adjustment of post-antibody change cortisol concentrations using a conversion formula.
Main Results:
- The area under the curve for T8 cortisol remained high (0.96) both before and after the antibody change.
- The optimal cutoff for diagnosing CS shifted from > 35.9 nmol/L (old antibody) to > 33.1 nmol/L (new antibody).
- The new cutoff demonstrated high sensitivity (92.5%) and specificity (95.0%) for CS diagnosis.
Conclusions:
- The optimal T8 cortisol cutoff for diagnosing canine Cushing syndrome after the antibody change is > 33.1 nmol/L (1.2 μg/dL).
- This value is lower than the previously accepted cutoff of > 38.6 nmol/L (1.4 μg/dL).
- This study identifies a new, validated cutoff for the LDDST in dogs.
Objective:
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.
Methods:
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.
Results:
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.
Conclusions:
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).
Clinical Relevance:
New cutoff value of T8 cortisol has been identified for the diagnosis of CS.
Related Concept Videos
01:26Cushing Syndrome I: Introduction
01:19Cushing Syndrome II: Pathophysiology