Activated Clotting Time ACT

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Activated Clotting Time as a measure of blood coagulation, useful for assessing the risk of clinical bleeding in patients who are not bleeding at the time they are tested. Introduced as a replacement for prothrombin time and partial thromboplastin time, ACT is a relatively simple test that can be performed by many hospital-based laboratories using currently available equipment.

ACT measures both intrinsic and extrinsic pathway clotting factors. It is normally expressed as a ratio of the patient's ACT to a normal value that has been determined for each assay. Normal ACTs are about twice those of prothrombin time and partial thromboplastin times because intrinsic and extrinsic pathways are important in both normal coagulation and pathological bleeding. As a result, an abnormal ACT may diagnose a problem with either normal or pathological clotting.

Problems with ACT are mostly related to the use of heparin and other anticoagulants (for example warfarin). In patients who are receiving these medications, it is important that the results of ACT be interpreted in conjunction with the results of the APTT because ACT does not measure a specific heparin-induced anticoagulant pathway, and therefore may be misleading in some situations. While we offer ACT tests, it is important to understand the broader implications of blood testing in our article on "Understanding blood tests".

ACT is also useful for monitoring patients who are being treated for bleeding disorders such as hemophilia or von Willebrand disease. In these cases, it is important to monitor the patient's response to treatment.