Introduction: Short synacthen test (SST) is of value in assessing the adequacy of hypothalamicpituitaryadrenal axis (HPA). Although it is extensively used, it is unclear at what morning cortisol concentration one should consider performing the SST.
Methods: Retrospective observational study of consecutively performed SST (250 μg) between January 2009 and March 2010. Plasma cortisol was measured by enzyme immunoassay (Siemens Advia, Siemens plc, UK). A cut-off value of ≥550 nmol/l was used to differentiate adequate from inadequate response.
Results: 76 consecutively reviewed endocrine out patients (females n (%), 53 (70)), median age (interquartile range) 46.5 (35.055.7) years underwent SST. Baseline and 30min cortisol levels were 367 (266502) and 745 (640845) nmol/l respectively. Cortisol levels increased by 318 (219436) nmol/l. There was no correlation between age and rise in cortisol level from baseline. There was no significant difference between baseline and 30 min cortisol and rise in cortisol between males and females. 50% (7/14) patients with baseline cortisol <250 nmol/l had 30 min cortisol <550 nmol/l. No patient with a baseline cortisol >250 nmol/l had 30 min cortisol <550 nmol/l (sensitivity 57%, specificity 91%). 92% (70/76) underwent the test before 1100 h. 30 min (not baseline) cortisol levels of the SST done after 1100 h versus before 1100 h were significantly lower, 751 (642851) vs 642 (524699); P=0.04.
Conclusion: Morning (before 1100 h) cortisol levels of >250 nmol/l seems to be associated with adequate adrenocortical reserve. In such situations it may not be necessary to do a SST, unless clinical suspicion is high. Further studies are needed to determine the effect of timing of SST on its sensitivity and specificity.