LAUSR

Background In the evaluation for hypercortisolism (Cushing’s syndrome), the 1 mg overnight dexamethasone suppression test has an important role, but false-positive results can occur due to low serum dexamethasone. Given the high intraindividual reproducibility of post-dexamethasone suppression test serum cortisol concentrations, we investigated the chance of success of repeating a non-suppressed dexamethasone suppression test if serum dexamethasone is low. Methods We retrospectively analysed the results of 1901 consecutive dexamethasone suppression tests performed in our laboratory from February 2011 to November 2018. Serum dexamethasone and cortisol were measured by LC-MS/MS, and both were reported. The 2.5 and 5th percentiles of serum dexamethasone in suppressed dexamethasone suppression tests were investigated as cut-off value. Then, we retrospectively determined the success rate of repeating an initial, non-suppressed dexamethasone suppression test in 131 patients, stratified by initial serum dexamethasone. Results At serum dexamethasone concentrations between the 2.5 and 5th percentiles (3.2–3.9 nmol/L), significantly more non-suppressed dexamethasone suppression tests were observed (27/67) than in the control group of 1357 tests having serum dexamethasone ⩾6 nmol/L (40% vs. 30%, P = 0.047), indicating that 3.9 nmol/L is the better cut-off. Overall, 40% of non-suppressed dexamethasone suppression tests were repeated, but repeat testing was performed more often when serum dexamethasone was low. In patients who had initial serum dexamethasone below the cut-off of 3.9 nmol/L, a significantly higher chance of having a suppressed repeat dexamethasone suppression test was observed compared to the control group: 57% (31/54) vs. 26% (15/57), P = 0.001. Conclusions Measuring and reporting serum dexamethasone in dexamethasone suppression tests have added value for the selection of patients who might benefit from a repeat dexamethasone suppression test. We suggest a cut-off for serum dexamethasone of ⩾3.9 nmol/L.