The activated clotting time (ACT) test evaluates coagulation status. The ACT responds linearly to heparin level changes and responds to wider ranges of heparin concentrations than does the aPTT. The ACT, however, assays overall coagulation activity. Therefore, prolonged values may not be exclusively the result of heparin.
The ACT can be a bedside procedure and requires only 0.4 mL of blood. Heparin infusion or reversal with protamine can then be titrated almost immediately according to the ACT results. ACT also is routinely used during dialysis, coronary artery bypass procedures, arteriograms, and percutaneous transluminal coronary arteriography. This test is hard to standardize, and no controls are available; therefore, it is used with caution mainly in cardiac surgery. The results are backed up by the aPTT.