Endocrine Abstracts

Background: The standard synacthen test (SST) is commonly utilised to interrogate the hypothalamo-pituitary-adrenal (HPA) axis in children. It comprises baseline and 30-minute serum cortisol concentrations (SCC), after injecting synthetic adrenocorticotropic hormone (ACTH)[1–24]. There is debate regarding the utility of a 60-minute SCC in the SST protocol with most studies to date conducted in adults.

Aim: To assess the utility of a 60-minute SCC in the SST to diagnose adrenal insufficiency (AI) in children.

Method: A retrospective, single-centre study was conducted at a tertiary paediatric hospital. Anonymised data from April 2019-November 2021 was analysed. Both 2-sample SST (baseline and 30-minute SCC), and 3-sample SST (baseline, 30- and 60-minute SCC) were conducted during this period due to local protocol variation. SCC was measured by immunoassay (Siemens IMMULITE-2000XPI analyser).

Results: Data from 160 patients was analysed (mean age 6.3years [0.0008-24.9]; 86 female). 54% were steroid naïve. 93 patients underwent a 2-sample SST and 67 underwent a 3-sample SST. There was a positive correlation between 30- and 60-minute SCC (r=0.96, P< 0.001) and no patient with a 30-minute “optimal” response (SCC >500nmol/l) had a “sub-optimal” response (SCC<500nmol/l) at 60 minutes. 52% (n=83) had a sub-optimal response at 30 mins and the baseline SCC was not predictive of this. 13.4% of those who achieved a SCC>500nmol/l at 60 minutes were classified as a “delayed pass” (SCC <500nmol/l at 30-minutes).

Conclusion: Results from this paediatric cohort suggest there is utility in the inclusion of 0-, 30-, and 60-minute SCC as part of the SST. The 30-minute SCC measurements have been validated against the gold-standard insulin tolerance test, however relying on this alone may over-diagnose AI, resulting in unnecessary regular treatment with exogenous steroid. The SCC at 30 minutes, produced in response to synacthen stimulation, is an indicator of the readily releasable pool of cortisol. The SCC at 60 minute is indicative of cortisol synthesis, dependent on ACTH signalling. Relying on a 60-minute SCC alone may result in a diagnosis of a “normal” HPA axis, when in fact glucocorticoid support during sickness/stress may be prudent if the 30-minute SCC is found to be sub-optimal.