Activated Clotting Time (ACT)

The APTT is preferred to the ACT for monitoring standard heparin therapy (for treatment of venous thromboembolism, unstable angina, myocardial infarction etc.), because it shows significantly better precision and is less subject to technical variability. On the other hand, the ACT is the recommended test for monitoring heparin in interventional cardiology procedures. The ACT correlates with the anticoagulant effect of heparin at the higher heparin levels used in these procedures. The APTT is unsuitable at this level of heparinization, as it is frequently prolonged beyond the measurable range.

The ACT is a test of whole blood that uses a strong contact activator of the intrinsic coagulation pathway, either celite or kaolin. As a result, it is linearly responsive to the high concentrations of heparin used during bypass (1-5 U/mL). The usual monitoring protocol consists of a baseline ACT followed by a bolus of heparin. The baseline ACT should be shorter than 200 seconds and the postheparinization ACT should be longer than 450 seconds prior to the onset of bypass. During bypass, the ACT is repeated every 15 to 20 minutes; if the result is less than 450 seconds, additional heparin is administered. The ACT is designed to be prolonged about 100 seconds above baseline for each unit per milliliter of heparin concentration in a typical patient. At the completion of CPB, heparin is neutralized with protamine and the ACT is performed again to ensure that it has returned to baseline levels.