Introduction: Current guidelines from The Endocrine Society indicate that 2 different tests from urinary free cortisol (UFC) measurement, salivary cortisol (SC) measurement, low-dose dexamethasone suppression test (LDDST) or overnight dexamethasone suppression test (ODST) are required to diagnose Cushings syndrome (CS). The low-dose dexamethasone test (LDDST) is often used as the confirmatory test when diagnosing CS. The overnight dexamethasone suppression test (ODST) is often utilised as a screening test for CS, and is faster, cheaper, and easier to perform than the LDDST. This study aims to determine the level of concordance between the LDDST and ODST.
Subjects and Methods: This is a retrospective cohort study, analysing results from September 2005 - April 2017 in two tertiary care centres, for patients who had undergone an ODST, UFC measurement, or late-night SC measurement, in addition to a LDDST. Concordance and Cohens kappa coefficient were calculated for comparison of all tests. The level of correlation between final serum cortisol concentration of the LDDST and ODST was determined using Pearsons correlation coefficient (P≤0.05).
Results: 43 patients were included in the study. Concordance was shown between all tests. The LDDST was 89.2% concordant with the ODST, with a kappa coefficient of 0.549. The final serum cortisol concentrations of the LDDST and ODST showed a significant positive correlation, with a Pearsons correlation coefficient of 0.82 (P≤0.05).
Conclusion: The ODST has the potential to replace the LDDST as the confirmatory test for CS diagnosis, especially when existing literature regarding sensitivity and specificity is reviewed.