Influence of sex, body mass index and age on the results of the 2-mg overnight dexamethasone suppression test

Abstract Background: The overnight dexamethasone suppression test (DST) is routinely used in establishing the diagnosis of Cushing’s syndrome. However, factors such as variable resorption and increased metabolism of dexamethasone (DXMS) could lead to false positive results. The aim of our study was to evaluate the influence of sex, body mass index (BMI) and age on the DXMS metabolism. Methods: In total, 183 subjects were enrolled in a regular 2 mg DST. Patients with Cushing’s syndrome, depression, renal or hepatic insufficiency and patients treated with liver enzyme modulating pharmaceuticals were excluded from this study, so that 72 patients and 66 healthy subjects were analyzed. Nocturnal blood withdrawals were performed in 11 adult subjects at 1:00 AM, 3:00 AM, 5:00 AM, 7:00 AM, 8:00 AM to evaluate the kinetic of DXMS and cortisol. Results: In the DST DXMS levels demonstrated a high variation at 8:00 AM, that was inversely correlated with BMI (r=−0.24, p=0.045). Furthermore, DXMS levels determined in patients with a BMI<30 kg/m2 (n=29) were significantly higher than values of obese patients with a BMI>30 kg/m2 (n=43) (1.66±0.71 ng/mL vs. 1.31±0.57 ng/mL, p=0.026). Cortisol levels, measured at the same time, were neither correlated with DXMS nor with BMI values. In the kinetic study, maximal DXMS concentration was measured at approximately 3:00 AM. At 8:00 AM, median of DXMS was reduced to 59.1% and was correlated directly with the DXMS AUC (r=0.75, p=0.013). However, there was no correlation between the cortisol levels at 8:00 AM and the nocturnal DXMS concentrations. Conclusions: In the DST BMI of individual patients could modulate the resorption rate and metabolism of DXMS but appears to have no impact on cortisol levels. Therefore, obesity should not be a cause of falsely positive results in the DST.