Activated clotting time (ACT) vs aPTT

Activated clotting time (ACT), also known as activated coagulation time is a test of coagulation.^[1]

The ACT test can be used to monitor anticoagulation effects, such as high-dose heparin before, during, and shortly after procedures that require intenseanticoagulant administration, such as cardiac bypass, cardiac angioplasty, thrombolysis, extra-corporeal membrane oxygenation (ECMO) and continuous dialysis.^[1]It measures the seconds needed for whole blood to clot upon exposure to an activator of an intrinsic pathway by the addition of factor XII activators.

 It is ordered in situations where the PTT test is not clinically useful (i.e., high-dose heparin therapy or presence of lupus anticoagulant). The ACT test is sometimes used, along with the PTT, to monitor the therapeutic effect of a direct thrombin inhibitor, such as argatroban or bivalirudin. 

The ACT is measured in seconds: the longer the time to clot, the higher the degree of clotting inhibition. During surgery, the ACT is kept above a lower time limit, a limit at which most people will not form blood clots. There is no widespread agreement of exactly what this lower limit should be. It will vary from hospital to hospital and depends to some degree on the method used to determine ACT. It is important to evaluate how the person is responding to this ACT lower limit and to the amount of heparin they are being given. The amount of heparin needed to reach and maintain a certain ACT (for instance, 300 seconds) will vary as will the body's clotting potential at that ACT. If there are clotting or bleeding problems, the dosages and ACT may need to be adjusted accordingly. After surgery, the ACT may be maintained within a narrow range (for instance, 175 – 225 seconds) until the person has stabilized.

ACT and PTT results are not interchangeable. In the area where they overlap (upper measurements of PTT and lower levels of ACT), they have poor correlation. ACT and PTT results should be evaluated independently. It is better to determine someone's heparin anticoagulant requirements, stabilize them, and then change the monitoring tool.

The ACT may be influenced by a person's platelet count and platelet function. Platelets that are activated during surgery often become dysfunctional, and both surgery and heparin can sometimes cause platelet numbers to decrease (thrombocytopenia). The temperature of the blood may also affect ACT results – the blood tends to cool during surgery as it is mechanically filtered and oxygenated. Acquired and inherited conditions such as coagulation factor deficiencies may also affect ACT results.

Both the aPTT and ACT are clinically useful for the monitoring of heparin therapy. The aPTT is used more frequently for routine monitoring; the ACT is used in specialized situations requiring large heparin doses. The ACT is typically performed at bedside and is capable of yielding results rapidly and perhaps at a lower cost than an aPTT performed by a central laboratory.