Synacthen stimulation test (SST) is a dynamic assessment of adrenal insufficiency and the 30-minute serum cortisol of >550 nmol/L is widely accepted as a diagnostic cut-off for adequate synacthen response. However, recent studies have demonstrated variability in serum cortisol concentrations measured by different automated immunoassay platforms.

At PathWest QEII, serum cortisol has been analysed on the Abbott Architect since September 2008. The running average median of 30-minute stimulated cortisol has fallen significantly, from 670 nmol/L to 540 nmol/L. There has been no notable drift on the Centaur.

All SSTs performed since January 2012 were analysed on Abbott Architect, Roche E170, Siemens Centaur and Immulite automated platforms. To date, there are 110 SST with complete data including 26 patients (24%) with a history of pituitary tumour or surgery. The median 30-minute serum cortisol in nmol/L was 540 on Abbott, 666 on Immulite, 599 on Centaur and 706 on E170. Using >550 nmol/L diagnostic cut-off, 45% patients would pass on Abbott, 77% on Immulite, 64% on Centaur and 80% on E170.

Using the suggested method-specific 30-minute serum cortisol cut-off derived from 165 healthy volunteers by El-Farhan et al: 81% patients would “pass” with Abbott (>430.4 nmol/L), Centaur (>498.7 nmol/L), and E170 (>573.5 nmol/L and >524.4 nmol/L for males and females respectively). Using >474.4 nmol/L on Immulite, 85% patients would have adequate response.

Conclusion: There are clearly differences in measured cortisol response to synacthen stimulation between various immunoassay platforms. We recommend adopting 30-minute serum cortisol of >430 nmol/L for defining adequate synacthen response when Abbott Architect method is used.