Activated Clotting Time (ACT)
The ACT is a standardized measurement of the patients procoagulation or anticoagulation level. The unit of measurement is the time (in seconds) that is required for an ACT counter to detect the formation of blood clot in a 2-3 milliliter sample of whole blood. Normal ACT levels range from 100-130 seconds, however cardiopulmonary bypass requires anticoagulation levels of 480+ seconds. The ACT is affected by temperature, fibrinogen, and platelet concentrations as well as platelet function. Patients may have up to a twelve-fold variability in the reaction, due to different individual rates for metabolizing heparin. The ACT provides a crude guide to an individual’s response. The activator utilized stimulates the intrinsic cascade (Kaolin and cellite) or the final common pathway (thrombin). Abnormalities in the platelet function or fibrinogen levels must be extreme before the test is considerably prolonged. Hypothermia and hemodilution are factors associated with CPB and directly affect the ACT. Hypothermia has the most profound effect, in terms of prolonging intraoperative ACT’s.