Endocrine Abstracts

Background: The 1 mg overnight dexamethasone suppression test (ONDST) is recommended as a first-line test for the investigation of Cushing Syndrome. Measurement of dexamethasone alongside cortisol in a 9am serum sample has been credited with improving diagnostic sensitivity of the test. Previous studies have also looked at the utility of salivary dexamethasone and have observed poor correlation with serum dexamethasone. Herein we introduce the concept of measuring 11-dehydrodexamethasone: a dexamethasone metabolite produced in the salivary duct through metabolism with 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2).

Methods: Paired 9am serum and saliva samples (n=44) were collected post ONDST. The saliva samples were analysed utilising a novel in-house LC-MS/MS assay which measured cortisol, cortisone, dexamethasone and 11-dehydrodexamethasone. Serum cortisol and dexamethasone had previously been analysed separately by LC-MS/MS. Results were compared and correlations were assessed using Pearson regression.

Results: Serum and salivary dexamethasone exhibited poor correlation (R-squared = 0.16), supporting what has been observed previously. Salivary dexamethasone and 11-dehydrodexamethasone also showed poor correlation (R-squared = 0.19). Serum dexamethasone and salivary 11-dehydrodexamethasone yielded a positive correlation with an R-squared of 0.65. However, at higher concentrations the correlation appeared non-linear.

Conclusion: Correlation of serum dexamethasone with 11-dehydrodexamethasone is a novel finding, which to the author’s knowledge, has not been previously established or investigated. It is hypothesised that the non-linear correlation might be due to saturation kinetics of the enzyme 11B-HSD2 in the salivary duct, a similar relationship was observed when comparison of salivary cortisol and cortisone was assessed. The development of this LC-MS/MS assay for salivary measurement of 11-dehydrodexamethasone opens the possibility for a fully remote DST, whereby patients could administer the drug and take a 9am saliva sample. Further sample comparisons are required to develop a 11-dehydrodexamethasone cut-off which equates to a serum dexamethasone concentration of 3.3 nmol/l.