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Activated Clotting Time (ACT)

The activated whole blood clotting time is a rapid bedside test for monitoring heparin anticoagulation. Changes in the ACT are directly and linearly proportional to the concentration of heparin. The degree of prolongation of the ACT is a useful index of a patient's level of anticoagulation, and the heparin dose may be adjusted according to the result. Other factors that may affect the ACT include hemodilution, hypothermia, cardioplegic solutions, platelet dysfunction, hypofibrinogenemia, other coagulopathies and certain drugs. ACT is prolonged in patients with antiphospholipid antibodies and may not demonstrate a linear response to heparin. If unexpected test results are obtained, more specific coagulation tests should be performed for further investigation.

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 designed to be prolonged about 100 seconds above baseline for each unit per milliliter of heparin concentration in a typical patient. For example, heparin concentrations between 1 and 3 units/mL give ACT results ranging between 130 and 350 seconds.

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. 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.

Other applications and ACT target ranges using ACT with Celite activation are listed in the following table.

Reference range is 99 - 130 seconds.

Specimen requirement is 2 mL of whole blood collected in a celite activated glass vacutainer tube, mixed, and immediately inserted into the instrument.