Determining the utility of the 60 min cortisol measurement in the short synacthen test.

OBJECTIVE: Despite the widespread use of the short synacthen test (SST), there remains no clear consensus on sampling times for the measurement of serum cortisol that best determines adrenal reserve. We set out to establish whether there is any value in measuring serum cortisol at 60 min following administration of synacthen.
DESIGN: Retrospective data analysis of 500 SST results measuring 0, 30 and 60 min cortisol levels after administration of 250 μg of synacthen at 2 large urban National Health Teaching Hospitals in the UK. PATIENTS AND MEASUREMENTS: Individuals thought to have primary or secondary adrenal insufficiency given 250 μg of synacthen. MEASUREMENTS: Serum cortisol levels measured at 0, 30 and 60 min, looking to see how many people who had adrenal insufficiency at the 30 min sample but in whom the 60 min sample showed adequate adrenal reserve.
RESULTS: The results from 384 people were analysed. A total of 276 had normal responses at 30 min and also at 60 min. A sum of 33 individuals had 'insufficient' (i.e., <550 nmol/l) 30 min cortisol levels, rising to ≥ 550 nmol/l at the 60 min test. All 75 individuals who were insufficient at 60 min were also insufficient at 30 min. No individuals passed (≥550 nmol/l) at 30 min and then failed (<550 nmol/l) at 60 min.
CONCLUSIONS: These results suggest that a significant proportion of people undergoing a SST may be inappropriately diagnosed as having adrenal insufficiency if the 60 min sample is not measured. We suggest that the 60 min sample is measured in all individuals having a SST to prevent unnecessary over-diagnosis of adrenal insufficiency.