Basal Cortisol as a Predictor of Passing Short Synacthen Test and the Utility of the 60-Minute Poststimulation Cortisol.

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

This work aimed to identify the BC threshold predictive of AS and evaluate the utility of 30- and 60-minute stimulated cortisol measurements in response to SST.

METHODS

This research took place at a public tertiary pathology service (PathWest) in Western Australia. Of 1216 SSTs performed, 704 adult results were eligible. Exclusions were based on incomplete data or protocol deviations. Intervention was intramuscular Synacthen 250 mcg with cortisol measured at baseline, 30, and 60 minutes. AS was defined as serum cortisol greater than 430 nmol/L at 30 minutes and/or greater than 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 of 335 nmol/L or greater predicted AS at either 30 or 60 minutes. In a subgroup with pituitary pathology, a lower threshold of 240 nmol/L or greater maintained 95% specificity. For samples collected after 0945 hours, BC of 251 nmol/L or greater predicted AS. Applying 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.

CONCLUSION

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 poststimulation cortisol identified a small group who would have failed SST using a single poststimulation measurement.