Endocrine Abstracts

The dexamethasone suppression test (DST) is a common screening test in Cushing’s syndrome but associated with false positive result in 10-20%. One cause of false positivity is inadequate dexamethasone absorption. An assay to measure serum dexamethasone concentration can be used to validate the accuracy of the DST result.

Aims: To audit 1) Use of the dexamethasone assay in DSTs 2) How frequently dexamethasone levels are inadequate 3) Causes of inadequate dexamethasone levels

Methods: The records of 178 patients in our centre who underwent ONDST/LDDST including dexamethasone level results between August 2021 and June 2023 were retrospectively analysed. Dexamethasone levels were included as part of the screening test rather than for a specific indication in patients.

Results: 21 patients (11.8%) had low dexamethasone level (< 3.0 nmol/l) as defined by the Wythenshawe Lab and based on liquid chromatography and tandem mass spectrometry. Of these 21 patients, 2 patients (9.5%) were on an interfering medication (carbamazepine); 2 (9.5%) had gastric absorption issues; 6 patients (28.5%) had repeat DST which was subsequently negative and it was deemed that compliance was the most likely cause of the initial false positive result (and low dexamethasone level). BMI was not associated with dexamethasone assay level result (P=0.16).

Conclusion: We find 11.8% DST results to be false positive due to inadequate dexamethasone absorption as per dexamethasone assay results. Of these, 28.5% are subsequently negative on repeat testing (with adequate dexamethasone level) and initial false positive was likely due to compliance issue. To our knowledge this is the first study to demonstrate use of the dexamethasone level assay to identify absorption or compliance issues as cause of false positive dexamethasone screening test results. It emphasises the importance of considering possibility of false positive results and secondly, need for clear instructions to patients to ensure compliance.