Introduction
Adrenal Insufficiency (AI), especially iatrogenic-AI, is a treatable cause of mortality. The difficulty in obtaining 9am cortisol levels means samples are taken at suboptimal times, including a substantial proportion in the afternoon. Low afternoon cortisol levels often provoke short Synacthen Tests (SSTs). It is important that this does not lead to patients misdiagnosed with AI, exposing them to the excess mortality and morbidity of inappropriate steroid replacement therapy.

Methods
This retrospective study collected 60,178 cortisol results. Medical records, including subsequent SSTs of initial cortisol results measured after midday were reviewed.

Results
ROC analysis (AUC- 0.89) on 6531 suitable cortisol values showed that a limit of <201.5nmol/L achieved a sensitivity and specificity of 95.6% and 72.6%, whilst a limit of <234nmol/L had a sensitivity of 100% and a specificity of 59.5%. Out of 670 SSTs, 628 patients passed. Of these, 140 would have otherwise failed if only their 30-minute cortisol was assessed without the 60-minute value.

A 30-minute and 60-minute SST cortisol cut-off of 366.5nmol/L and 418.5nmol/L respectively, can achieve a sensitivity of >95% on the Abbott analyser platform.

Conclusion
An afternoon cortisol >234nmol/L excludes AI on Abbott analyser platforms. In patients who have an afternoon cortisol <234nmol/L, including both a 30-minute and a 60-minute SST cortisol values prevents unnecessary glucocorticoid replacement therapy in 22.3% of individuals in this study. The Abbott analyser SST cortisol cut-offs used to define AI should be 366.5nmol/L and 418.5nmol/L at 30- and 60-minutes respectively. All patients remained well subsequently with at least one year longitudinal follow up.