The short Synacthen test (SST) is used to evaluate adrenal glucocorticoid secretion in response to synthetic ACTH (also known as tetracosactide or Synacthen). Traditionally, after a baseline cortisol and ACTH, two plasma cortisol samples have been taken after i.v. or i.m. administration of Synacthen 250 μg, one at 30 min and one at 60 min. However practice varies and some physicians only take one cortisol measurement at 30 min.
Hypothesis: A protocol involving one assessment of plasma cortisol levels post Synacthen administration is as effective at confirming hypoadrenalism as the standard two sample protocol.
We studied 101 patients who had undergone a SST at Hammersmith Hospital between January 2009 and Nov 2010. We accepted a post Synacthen cortisol level of ≥550 nmol/l (≥440 nmol/L on new Abbott immunoassay as cortisol levels on this assay are reported to be 20% lower) as evidence of an intact HPA axis.
Overall 88% showed an adequate response (ie passed) on SST. Of this group 100% passed the SST based on the 60 min cortisol value alone. 95% passed if the 30 min cortisol value alone was assessed, giving a false negative (i.e. failed test) rate of 5%. Reduction of the upper range of normal for the 30 min time point to ≥444 nmol/l (≥355 nmol/l on Abbott immunoassay) provided a sensitivity of 100% with respect to the full protocol results, and did not provide any false negatives or false positives.
We conclude that a modified SST protocol could be based on either a sole 60 min post Synacthen cortisol level assessment, or a sole 30 min post Synacthen cortisol assessment with an adjusted lower cut off at this time point. A retrospective audit of 100 further SSTs is planned to check the performance of both these strategies.