Predicting the Outcome of Short Synacthen Test Based on Baseline Cortisol Levels: A Single-Centered Retrospective Cohort Study at a Tertiary Care Hospital in Sri Lanka

Short Synacthen Test (SST), a standard diagnostic test to confirm Adrenal insufficiency (AI), involves substantial expenses. Aim to assess the predictive value of baseline cortisol levels for SST outcomes and establish baseline cut-off levels to confirm AI and minimize the necessity of SST. SST data from 2019 to 2024 at National Hospital Kandy, Sri Lanka, were obtained retrospectively. A peak cortisol ≥500nmol/L at 30 or 60-minutes post-SST was considered as a normal adrenal reserve, whereas failure indicated AI. Pearson's correlation assessed baseline and post-SST cortisol. Receiver operating characteristic (ROC) curve analysis evaluated the SST outcomes assessing sensitivity, specificity, and area under the curve (AUC). 307 patients were enrolled, and 63.19% exhibited a failed SST response. Baseline cortisol positively correlated with post-SST cortisol at 30-minutes (r=0.74, p<0.05) and 60-minutes (r=0.68, p<0.05) with a good AUC for both 30-minutes (AUC=0.855) and 60-minutes (AUC=0.829). ROC curves were utilized to derive cut-offs for baseline cortisol levels predicting SST outcomes. At 30-minutes, baseline cortisol <135nmol/L suggests AI (100%sensitivity, 44%specificity), and >381.5nmol/L indicates normal reserve (100%specificity, 21.8%sensitivity). Similarly at 60-minutes, baseline cortisol <75.3nmol/L suggests AI (100%sensitivity, 19.7%specificity), and >357nmol/L indicates normal reserve (100%specificity, 16.8%sensitivity). Applying these cut-offs could avoid 41.69% (30-minutes) or 19% (60-minutes) of total SSTs, excluding AI and normal reserve. 30-minutes SST cortisol correlates more strongly with baseline cortisol, showing a higher r-value and AUC. Hence, 30-minutes provides better cut-offs with higher sensitivity and specificity. Patients with baseline cortisol between 135-381nmol/L, undergo SST with only a 30-minutes cortisol.