Short synacthen test (SST) is the most widely used dynamic test of hypothalamic-pituitary-adrenal (HPA) axis function. Its simple to conduct but requires nursing time and is relatively costly given 15-fold price increase in synacthen since 2015. We audited our SST use with the aim of reviewing clinical indications for testing and identifying useful predictors of test outcome. Baseline referral, clinical and biochemical data were retrospectively collected for individuals undergoing SST between June 2018 and December 2020. Binomial logistic regression and ROC curves analysis were performed using SPSS27. Nine hundred SSTs were performed in 767 patients (female 61%, mean age 51 ± 19 years) and comprised patients with known pituitary disease (60.56%), GP-referrals (25.67%) with possible symptoms of adrenal insufficiency (AI) and referrals queried tertiary AI (9.44%). From 900 SSTs, 658 (73.11%) resulted in a pass (30minute cortisol ≥430nmol/l) with 90.47% of GP referrals and 45.88% referrals for possible tertiary AI passing the SST. No specific symptom, clinical or biochemical pre-SST parameter could predict test outcome. However, a pre-test 9am cortisol≥273 nmol/l in the GP referral cohort predicted a definite pass with 100% specificity (ROC AUC 0.804, P < 0.001). Delta cortisol level (SST baseline cortisol-30mins cortisol) was a better predictor (ROC AUC 0.873, P < 0.001) of SST outcome in the suspected tertiary AI group when compared to pre-test 9am cortisol. In keeping with the non-specific clinical phenotype of AI, common symptoms, clinical and biochemical variables are unhelpful predictors of SST outcome. A pre-test 9am cortisol is a useful predictor and we propose to lower our 9am cortisol threshold for SSTs where pre-test probability is low. Tertiary AI is common and the pre-test 9am cortisol is less predictive in this group than delta cortisol, which could help identify which patients are more likely to later recover HPA axis function.
08 Nov 2021 - 10 Nov 2021