In November 2020, Siemens Healthineers implemented a permanent antibody change on the cortisol assay used on the IMMULITE® 2000/XPi Immunoassay Systems. This is the cortisol assay performed at The CityU Veterinary Diagnostic Laboratory (CityU VDL). The new antibody has shown different cross-reactivity to canine cortisol. An initial review by the European Society of Veterinary Endocrinology Endocrine Quality Assurance Scheme (ESVE-EQA) suggests that canine cortisol results are lower (average bias -23% in canine serum). The effect appears to be more marked in canine urine cortisol (-70%).

After being alerted by the ESVE-EQA, the manufacturer has incorporated into the assay a formula derived from mathematical adjustment factors to counteract the impact on canine serum cortisol. With this adjustment formula, the average bias decreases significantly.

The adjustment formula does not seem to work for the canine urine cortisol, however. Therefore, the ESVE and British Small Animal Veterinary Association (BSAVA) have recommend the laboratories implement new cut-off values for the interpretation of the urine cortisol to creatinine ratio (UCCR).

In early March 2021, the City University of Hong Kong Veterinary Diagnostic Laboratory (CityU VDL) started using the new cortisol assay. In-house analyses show good correlation (r = 0.99) between the previous and new antibodies for canine serum cortisol, with an average bias of 1.3%. Unlike that reported by the ESVE-EQA, our results suggest a small positive bias with the new assay. Veterinarians should be aware of the possible impact this bias may have on the results.

Our analyses revealed a good correlation (r = 0.97) for urine cortisol, but a significant average negative bias (-56%) despite the implementation of the adjustment formula into the system. Following the recommendations given by the ESVE and BSAVA, it was decided to adjust the interpretative guidelines of the UCCR. The cut-off value to rule out hyperadrenocorticism was decreased from the original <10x10^-6 to <3x10^-6.

Based on these data and recommendations by the ESVE, we advise that cortisol results that are close to a threshold be interpreted with caution. We remind clinicians that laboratory results should always be interpreted in light of clinical signs and findings.

The assay manufacturers stated that their long-term goal is to provide a Veterinary Use Only kit, IMMULITE 2000/XPi Canine Cortisol. The projected availability of this kit was January 2021. The CityU VDL has been in touch with the manufacturer, and at present the new assay appears not to be available in Hong Kong. We will keep veterinarians posted about any future developments.

For more information about the new cortisol assay you can visit the ESVE website:
https://www.esve.org/