An investigation of a new activated clotting time "MAX-ACT" in patients undergoing extracorporeal circulation

Activated clotting time (ACT) is a test used in the operating room for moni toring heparin effect. However, ACT does not correlate with heparin levels because of its lack of specificity for heparin and its variability during h ypothermia and hemodilution on cardiopulmonary bypass (CPB). A modified ACT using maximal activation of Factor XII, MAX-ACT (Actalyke MAX-ACT; Array M edical, Somerville, NJ), may be less variable and more closely related to h eparin levels. We compared MAX-ACT with ACT in 27 patients undergoing CPB. We measured ACT, MAX-ACT, temperature, and hematocrit at six time points: b aseline; postheparin; on CPB 30, 60, and 90 min; and postprotamine. Additio nally, we assessed anti-Factor Xa heparin activity and antithrombin III act ivity at four of these six time points. With institution of CPB and hemodil ution, MAX-ACT and ACT did not change significantly but had a tendency to i ncrease, whereas concomitant heparin levels decreased (P = 0.065). Neither test correlated with heparin levels. ACT and MAX-ACT did not differ during normothermia but did during hypothermia, and ACT was significantly longer t han MAX-ACT (P = 0.009). At the postheparin time point, ACT-heparin sensiti vity (defined as [ACT postheparin - ACT baseline]/ [heparin concentration p ostheparin - heparin concentration baseline]) was greater than MAX-ACT-hepa rin sensitivity (analogous calculation for MAX-ACT; 520 [266 - 9366] s . U- 1 . mL(-1) vs 468 [203 - 8833] s . U-1 . mL(-1); P = 0.022).