What is the activated clotting time (ACT)?

From the Guidelines

Activated clotting time (ACT) should be used to monitor high-dose heparin therapy during procedures requiring anticoagulation, with target ranges varying by procedure, such as 400-480 seconds for cardiopulmonary bypass and 250-300 seconds for percutaneous coronary interventions. The ACT is a point-of-care test that measures the time it takes for whole blood to clot after the addition of an activating agent, and it is preferred over activated partial thromboplastin time (aPTT) in high-dose heparin scenarios because aPTT becomes less reliable at high heparin concentrations 1.

Key Points

• The normal ACT range is typically 80-120 seconds, but target ranges vary by procedure 1
• The ACT works by activating the intrinsic coagulation pathway, and the time to clot formation directly correlates with the level of anticoagulation present in the sample 1
• To perform the test, a small blood sample (about 2 mL) is collected in a tube containing an activator like kaolin or celite, then placed in an automated device that measures the time until clot formation 1
• ACT results are available within minutes, allowing for rapid dose adjustments of heparin 1

Procedure-Specific Target Ranges

• Cardiopulmonary bypass: 400-480 seconds 1
• Percutaneous coronary interventions: 250-300 seconds 1

Clinical Considerations

• The use of ACT to monitor heparin therapy is supported by guidelines from the Society of Thoracic Surgeons, the Society of Cardiovascular Anesthesiologists, and the American Society of Extracorporeal Technology 1
• The choice of anticoagulant and dosing strategy may vary depending on the procedure and patient factors, such as the use of glycoprotein inhibitors or prior anticoagulant therapy 1

From the Research

Activated Clotting Time (ACT) Overview

• ACT is a test used to measure the time it takes for blood to clot, and it is commonly used to monitor heparin therapy in patients undergoing cardiac surgery or other medical procedures 2, 3, 4.
• The test is typically performed using a portable device, and the results are used to determine the adequacy of heparin reversal and to guide protamine sulfate administration 2, 3.

Use of ACT in Cardiac Surgery

• Studies have shown that ACT can be used to construct a two-point heparin dose-response curve, which can be used to estimate the residual heparin activity and guide protamine dosage 3.
• The use of ACT monitoring and heparin dose-response curves has been shown to reduce the amount of protamine required and minimize the risk of bleeding complications 3, 4.
• ACT has been used to monitor heparinization and its reversal with protamine in cardiac surgery, and it has been shown to result in reduced dosages of heparin and protamine and reduced intra-operative bleeding 4.

Reliability of ACT Measurements

• Studies have investigated the reliability of ACT measurements at different time points after heparin injection, and the results suggest that ACT measured 2 minutes after heparin injection is reliable and correlates well with mean 1-5 minute values 5.
• However, other studies have found that the correlation between ACT and activated partial thromboplastin time (aPTT) is poor, and that ACT cannot differentiate between low and therapeutic levels of anticoagulation 6.

Clinical Applications of ACT

• ACT is commonly used in cardiac surgery to monitor heparinization and guide protamine dosage, and it has been shown to be a reliable and useful tool in this setting 2, 3, 4.
• However, the use of ACT in other clinical settings, such as in critically ill patients, may be limited by the poor correlation between ACT and aPTT, and the inability of ACT to differentiate between low and therapeutic levels of anticoagulation 6.