Two-day low-dose dexamethasone suppression test more accurate than overnight 1-mg in women taking oral contraceptives

Late-night salivary cortisol (LSaC) and 24-h urinary free cortisol measurement, and overnight 1-mg dexamethasone suppression test (1 mg-DST) are the first-line screening tests recommended for Cushing's syndrome. Through elevations in the level of cortisol-binding globulin, oral contraceptive agents lead to increases in the total plasma cortisol concentration, yielding false-positive 1 mg-DST results.

To compare the accuracy of the overnight 1-mg DST and two-day low-dose DST (2d-DST) in female volunteers taking combined oestrogen-progestin oral contraceptives (COCs).

This prospective study enrolled 30 healthy participants. Their plasma cortisol response levels were compared after the 1-mg DST and 2d-DST and classified into three categories: normal (≤50 nmol/L), doubtful (51-138 nmol/L) and abnormal (>138 nmol/L). Salivary cortisol was also measured at late night and after the DSTs.

Following the 1-mg DST and 2d-DST, the plasma cortisol concentrations decreased to a median of 69 nmol/L and 37 nmol/L, respectively (p < 0.001). A statistically significant higher proportion of unclear or abnormal results were observed after the 1-mg DST (63%) than after the 2d-DST (27%) (p = 0.004). None of the values were >138 nmol/L after the 2d-DST, while 11% of them were abnormal after the 1-mg DST (p = 0.25). No LSaC value was abnormal.

Our results suggest that, when late-night salivary cortisol is not available, the 2d-DST could be a better screening option than the 1-mg DST for women taking oral contraceptive agents who are reluctant to stop them. This finding requires confirmation in those with a suspicion of hypercortisolism.