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

Abstract

Introduction

Short Synacthen Test (SST), a standard diagnostic test to confirm Adrenal insufficiency (AI), involves substantial expenses.

Objectives

This 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.

Methods

All 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).

Results

A 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).

Conclusions

Applying 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.