Endocrine Abstracts

Background: The overnight dexamethasone (DEX) suppression test is useful for the investigation of hypercortisolism, however several factors may influence its performance. Intestinal uptake of DEX, inactivation by conversion by CYP3A4 in the liver and renal clearance can all affect test variability. It is also known that several drugs can either reduce or accelerate CYP3A4 activity, thereby affecting blood DEX concentrations. Interpretation of the test would be greatly enhanced by knowledge of the plasma DEX concentration when the morning cortisol sample is taken. We describe a sensitive LC–MS/MS assay for the analysis of the low concentrations of DEX encountered during this test.

Methods: Samples were obtained at 0800–0900 h from 90 postmenopausal women who all had a 1 mg ONDST. 250 μl sample and 10 μl internal standard (d4-DEX) was extracted with 1 ml MTBE. After mixing, the supernatant was blown down and reconstituted in 80 μl 40% MeOH. Sample extract was injected onto a C18 column and the eluate measured on a Xevo TQ mass spectrometer. Dexamethasone was eluted with a linear methanolic gradient. Total run time was 2.5 min.

Results: The LLOQ was 0.5 nmol/l and recovery was 100% (range 98–103%). CV was <8% at 1.9 nmol/l. The mean (S.D.) dexamethasone level was 7.8 (3.6) nmol/l whilst the mean dexamethasone cortisol was 31 nmol/l (S.D. 33 nmol/l; range 11–317 nmol/l). 6/90 patients had a dexamethasone cortisol level of >50 nmol/l and 4/6 of these participants had dexamethasone levels <5.6 nmol/l.

Discussion: We have developed a highly sensitive method for the evaluation of DEX in morning serum samples after an ONDST. 67% of participants who failed the ONDST had low dexamethasone levels highlighting the possibility of a false positive test. The study suggests that simultaneous measurement of dexamethasone and cortisol may allow more accurate evaluation of dexamethasone suppression test results but greater numbers are needed to define precise dexamethasone cut-offs.