Endocrine Abstracts

Short synacthen tests (SST) are used to investigate patients with suspected HPA pathology and a rise of serum total cortisol > 550nmol/L is accepted as demonstrating sufficient cortisol reserve. However, cortisol is 80% bound to cortisol binding globulin (CBG) and therefore abnormal values of CBG can influence serum total cortisol used to interpret dynamic tests of the HPA axis. The free cortisol index (FCI) is a surrogate marker for serum free cortisol and is defined as serum total cortisol (nmol/L) / CBG (mg/L). We have previously shown that 30 healthy volunteers undergoing a SST had a FCI >12 although 43% had max serum total cortisol < 550 nmol/L. We have also showned that 30 patients following routine abdominal surgery all had FCI > 13. This study aims to compare serum total cortisol and FCI in patients undergoing SSTs to diagnose HPA axis insufficiency.

Data on 21 females and 8 males having a SST at 09: 00 and who had a max serum total cortisol < 600 nmol/L were prospectively collected. Serum total cortisol was measured on Roche ES700 and serum CBG by Biosource RIA. FCI was calculated by serum total cortisol / CBG.

Ten of the 29 patients (34%) had max serum total cortisol < 400nmol/L (range 30-392 nmol/L) and all had a FCI < 11 (range 1-11). 7 patients (24%) had max serum total cortisol > 550 nmol/L and all had FCI > 12 (ranging to 17).Twelve patients (41%) had max serum total cortisol between 400 and 550 nmol/L and of these, 2 patients had a FCI of 11 with maximum serum total cortisols of 442 and 495 nmol/L respectively, while the other 10 had FCI > 13 (ranging to 21), but maximum serum total cortisol ranged from 466-543 nmol/L.In patients with serum total cortisol < 600 nmol/L, our data suggests discrepancies between the serum total cortisol and FCI in 34%. The measurement of the FCI in patients undergoing SST for the diagnosis of HPA axis insufficiency may be a more reliable marker of HPA axis reserve than the measurement of serum total cortisol.