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.

IntroductionShort Synacthen Test (SST), a standard diagnostic test to confirm Adrenal insufficiency (AI), involves substantial expenses.ObjectivesThis study aimed to assess the predictive value of baseline Cortisol levels for SST outcomes and establish baseline cut-off levels for confirming AI to minimize the necessity of SST.MethodsAll SST data from 2019 to 2024 at National Hospital Kandy, Sri Lanka, were obtained retrospectively. A peak Cortisol ≥500 nmol/L at 30 or 60-min post-SST was considered as a normal adrenal reserve, whereas failure indicated AI. Pearson's correlation and Logistic Regression analysis assessed baseline and post-SST Cortisol at 30 and 60-min. A 2 × 2 table assesses test agreement. Receiver operating characteristic (ROC) curve analysis evaluated the SST outcomes at 30 and 60-min separately assessing sensitivity, specificity, and area under the curve (AUC).ResultsA total of 307 patients were enrolled, and 63.19% exhibited a failed SST response. Baseline Cortisol positively correlated with post-SST Cortisol at 30-min (r = 0.74, P < .05) and 60-min (r = 0.68, P < .05) with a good AUC for both 30 min (AUC = 0.855) and 60 min (AUC = 0.829). Baseline Cortisol demonstrated the higher odds ratio per unit (OR = 1.015 per nmol/L), indicating greater sensitivity to small changes. ROC curves were utilized to derive cut-offs for baseline Cortisol levels predicting SST outcomes. At 30-min, baseline Cortisol <135 nmol/L suggests AI (100% sensitivity, 44% specificity), and >381.5 nmol/L indicates normal adrenal reserve (100% specificity, 21.8% sensitivity). Similarly at 60-min, baseline Cortisol <75.3 nmol/L suggests AI (100% sensitivity, 19.7% specificity), and >357 nmol/L indicates normal adrenal reserve (100% specificity, 16.8% sensitivity).ConclusionsApplying these cut-offs could avoid 41.69% (30 min) or 19% (60 min) of total SSTs, excluding AI and normal adrenal reserve. 30-min SST Cortisol correlates more strongly with baseline Cortisol, showing a higher r-value, higher OR and AUC. Hence, 30-min provides better cut-offs with higher sensitivity and specificity minimizing need for SST. Patients with baseline Cortisol between 135 and 381 nmol/L can undergo SST with only a 30-min Cortisol measurement.