2 Clinic of Cardiac Surgery,

In severely ill patients low concentrations of the corti-costeroid binding globulin are typically found; the aim of this study was to quantify directly free bioactive cor-tisol concentrations in the sera of postoperative car-diosurgical patients. Serum samples of 12 consecutive patients undergoing aortocoronary bypass surgery taken preoperatively and on the postoperative days 1 to 4 were analyzed. Total serum cortisol was quanti-fied using liquid chromatography-tandem mass spec-trometry with an on-line sample extraction system and tri-deuterated cortisol as the internal standard, and free serum cortisol was measured after over-night equilibrium dialysis. Whereas on the first postopera-tive day, the median total serum cortisol concentration was approximately two-fold increased compared to preoperative samples (preoperatively, 245 nmol/l (interquartile range (IQR) 203–293 nmol/l); first post-operative day, 512 nmol/l (IQR 410–611 nmol/l)), me-dian dialyzable free cortisol concentration was almost seven-fold increased (preoperatively, 14.2 nmol/l (IQR 10.9–20.7 nmol/l); first postoperative day, 98.3 nmol/l (IQR 81.3–134 nmol/l)). On the fourth postoperative day, median free cortisol was still significantly in-creased compared to baseline sampling (p < 0.05), whereas median total cortisol was not. A median of 5.7 % (IQR 5.4–7.0%) of total cortisol was found as free cortisol on the preoperative day, 21.2 % (IQR 18.9– 23.5%) on the first postoperative day and 10.5 % (IQR 9.8–14.0%) on the fourth postoperative day. It is con-cluded that during the postoperative period the free-to-bound ratio of cortisol is highly variable and that during the acute phase response direct quantification of free bioactive cortisol concentrations seems to be biologically more appropriate than the measurement of total cortisol concentrations. Clin Chem Lab Me