Recalibration of thinking about adrenocortical function assessment: how the ‘random’ cortisol relates to the short synacthen test results

Background The short synacthen test (SST) is the most commonly performed investigation to assess adrenal function. Appropriate criteria for when an SST is performed are subject to debate. We investigated how random serum cortisol levels relate to SST response. Methods We examined random cortisol measurements taken between 04.40–23.55 p.m. results of SST baseline and 30-/60-min cortisol performed over 12 months (225 SSTs) at Salford Royal Hospital. Serum cortisol was measured on the Siemens Centaur Analyser. A 30–60-min cortisol concentration of ≥450 nmol/L defined a pass; 350–449 nmol/L defined borderline. Results Patients only proceeded to SST if random cortisol was <400 nmol/L. For those not on corticosteroids for at least 2 weeks, 42/43 (97.7%) cases with random cortisol concentration of ≥200 nmol/L had an SST ‘pass’. The relation was less clear with corticosteroid treatment (19/35 cases; 54%). For those not taking glucocorticoid treatment (including inhaled/topical corticosteroids) in the previous 2 weeks, 91.8% of SSTs were pass/2.7% borderline/5.5% fail. For those on steroids, 51.9% of SSTs were a pass/11.4% were borderline. In relation to the postsynacthen cortisol pass cut-off of ≥450 nmol/L, in 15/207 (7.2%) of cases, the 60-min cortisol was ≥450 nmol/L (adequate adrenocortical function), but 30-min cortisol was below this. In all cases where the 30-min cortisol did indicate a pass (i.e. was ≥450 nmol/L) the 60-min cortisol was also ≥450 nmol/L. Conclusion Our findings suggest that if the random cortisol level is ≥200 nmol/L, regardless of the time of day and the person was not taking corticosteroid treatment in the previous 2 weeks, SST may not be needed. Our data also suggests that 60-min cortisol retains utility.