**4.1 Activated clotting time**

The activated clotting time (ACT) is a whole blood test used at the bedside that provides immediate results [3, 6]. The ACT measures the time in seconds to form a fibrin clot after the addition of specific coagulation activators, thus it does not solely measure the effect of unfractionated heparin. It is a relatively inexpensive test that has been widely used to monitor heparin during cardiopulmonary bypass, but coagulation factor deficiencies, hemodilution, platelet function, and hypofibrinogenemia may affect the value [3, 19]. Due to these limitations in ACT, Baird and colleagues retrospectively reviewed 600 pediatric ECMO patients and found only a modest correlation between ACT and UFH dose suggesting that ACT may not be an accurate tool for monitoring UFH anticoagulation [20]. Bembea and colleagues found similar results to Baird with only a 42% correlation between target ACTs and anti-factor Xa [21].
