The short synacthen test (SST) is the first-line test of adrenal insufficiency. Different centres have varying protocols for cortisol response. At Derriford Hospital cortisol is measured at 0, 30 and 60 min post-synacthen 250 μg (i.m./i.v.). A normal response is judged with a measured cortisol >550 nmol/l.
A recent national audit questioned the bias created by the discrepancy between true cortisol (measured by mass spectrometry) and many laboratory assays. We carried out a retrospective study of the last 12 months of SST to assess the effect of a change of reference range for normal cortisol response from 550 to 465 nmol/l and to assess the utility of both a 30 and 60 min cortisol measurement.
Data was obtained for 157 SSTs carried out. 137 (87.3%) passed with a peak cortisol >550 nmol/l compared with 144 (91.7%) pass rate >465 nmol/l.
There were 7 (4.5%) tests in which a reduction of the target cortisol from >550 to >465 nmol/l would have resulted in a test being passed instead of failed. 2 results were from the same patient and 1 was discounted as only a 90 min sample was taken. Only 1 case lead to regular hydrocortisone therapy initiation. In two cases the patient was retested and subsequently passed the SST. In one case advice was given but no treatment was commenced. In the final case repeat testing was planned the following year.
Only 5/157 (3.2%) of the 60 min cortisol samples were lower than the 30 min cortisols (the overall SST result was unchanged in these cases).
It was concluded that the results of an SST always requires interpretation in view of the clinical scenario. Alteration of the reference range for cortisol would lead to an increased pass rate but might not change management greatly. The test should be repeated if the re is any doubt.