Basal cortisol as a predictor of passing Short Synacthen Test and the utility of the 60-minute post-stimulation cortisol

Abstract Context Short Synacthen Testing (SST) is resource intensive and may be unnecessary if adrenal sufficiency (AS) can be predicted reliably using basal cortisol (BC). Objective to identify the BC threshold predictive of AS and evaluate the utility of 30- and 60-minute stimulated cortisol measurements in response to SST. Setting Public tertiary pathology service (PathWest), Western Australia. Patients Of 1216 SSTs performed, 704 adult results were eligible. Exclusions were based on incomplete data or protocol deviations. Intervention Intramuscular Synacthen 250 mcg with cortisol measured at baseline, 30 and 60 minutes. Main Outcome Measures AS was defined as serum cortisol >430 nmol/L at 30 minutes and/or >500 nmol/L at 60 minutes. Receiver Operating Characteristic analysis identified BC thresholds predicting AS with 95% specificity. Results Of 704 SSTs included, 571 (81%) demonstrated AS. BC ≥335 nmol/L predicted AS at either 30- or 60-minutes. In a subgroup with pituitary pathology, a lower threshold of ≥240 nmol/L maintained 95% specificity. For samples collected after 0945h, BC ≥251 nmol/L predicted AS. Applynig these thresholds could have avoided 31.1% of SSTs. Nineteen patients (2.7%) passed SST solely at the 60-minute cortisol result with median cortisol of 420 nmol/L. Conclusions BC reliably predicts AS, avoiding unnecessary SST in a substantial number of patients. Adjusted thresholds for pituitary pathology and by time of day can enhance accuracy. Both 30- and 60-minute post-stimulation cortisol identified a small group who would have failed SST using a single post-stimulation measurement.