Heparin is used as an anticoagulant used to facilitate many medical procedures. Administration, however is complicated by the varying response from individual patients.

There are many factors that influence the efficacy of heparin. These include age, concomitant administration of oral anticoagulants or aspirin, prior use of heparin, platelet count and the presence or absence of a range of proteins.

Many attempts have been made to devise appropriate regimes for administration of heparin. In the presence of the  higher concentrations used during medical procedures, the activated clotting time (ACT) has been used as the standard measure of heparin activity.

There are three main activators, celite, kaolin, and glass beads used activators to initiate contact activation. The point-of-care ACT test has been the preferred test in catheterization labs and cardiac theatres because it has a number of advantages over laboratory tests .

Hortin and Augustin(1) have written: "There are other coagulation monitoring tests available; however these are usually specific and do not take into account the global picture of the entire clotting system. The standard coagulation tests (prothrombin time (PT), activated partial thromboplastin time, thrombin time (TT), and fibrinogen level) are plasma tests measuring plasma haemostasis and not patient haemostasis. The ACT measurement uses whole blood, thereby incorporating the importance of platelets and phospholipids in the role of coagulation. Many of the problems with the haemostatic system during ECLS are caused by the activation of platelets, which are not detected by standard tests. Because an ACT test is nonspecific there are many variables such as hypothermia, platelets, aprotinin, GP IIb/IIIa antagonists, haemodilution, etc. that can alter its results. For this reason it is important to gain an understanding as to how these variables interact for meaningful interpretation of the ACT test result."

1. Activated clotting time (ACT) Horton S, Augustin S. Methods Mol Biol. 2013;992:155-67. doi: 10.1007/978-1-62703-339-8_12. https://www.ncbi.nlm.nih.gov/pubmed/23546712

2. http://www.pocttesting.org/uploads/1/8/3/5/18355427/_istat_act1.pdf

Introduction