T his report provides an overview and update regarding the advantages, disadvantages, and clinical uses of the activated clotting time (ACT) and a description of the devices that are commercially available to perform the assay. A discussion of recent peer-reviewed literature is then presented, focusing on articles published after 2001.The ACT is a whole-blood clotting time. 1 To perform this test, clotting is initiated in a blood sample by an activator of the intrinsic pathway of coagulation. The assay is most useful for monitoring high-dose heparin anticoagulation, in particular during cardiopulmonary bypass surgery. The activated partial thromboplastin time (PTT), another test that is normally used to monitor heparin, cannot be used to monitor such high heparin levels because the PTT is unclottable at high concentrations of heparin. Although the ACT is virtually always performed at the patient's bedside, the PTT is usually performed in a laboratory. Consequently, the turnaround time is usually much shorter for the ACT than for the PTT. Therefore, the ACT can also be used when an immediate measure of heparin anticoagulation is required at the bedside, such as with extracorporeal membrane oxygenation (ECMO), hemodialysis, cardiac catheterization, and vascular surgery. Only in rare circumstances are samples for the ACT collected in anticoagulated tubes with subsequent performance of the test in a central laboratory. 2 Analyzers from different manufacturers use different methodologies, and the results from different instruments are not interchangeable. Therefore, reference ranges vary considerably between instruments; they usually are between 70 and 180 seconds. The target ACT also depends on the ACT device/method used. With the high doses of heparin used during cardiopulmonary bypass, an ACT goal of over 400-500 seconds (commonly .480 seconds) is often used, representing a mean heparin level of approximately 4-5 U/mL. 2,3 For other indications, the ACT goal is typically lower.There are several limitations associated with the ACT. The ACT is less precise than the PTT, and the assay does not correlate well with the PTT or with heparin concentration (antifactor Xa levels). 2,4-6 Results are influenced by a variety of variables, including platelet count, platelet function, factor deficiencies, ambient temperature, hypothermia, hemodilution, and lupus anticoagulants. If a lupus anticoagulant is present, a heparin assay (anti-factor Xa levels) can be used to determine the level of anticoagulation. Aprotinin, an agent that is often used during cardiopulmonary bypass to decrease bleeding, can affect the results of some ACT methods. Aprotinin prolongs celite-based ACTs but generally not kaolin-based ACTs. Thus, celite-based ACTs may overestimate the amount of heparin anticoagulation when aprotinin is present. Very high doses of aprotinin, such as following a large initial bolus, may also prolong kaolin-based ACTs.