Activated Clotting Time
- Author: Vadim Kostousov, MD; Chief Editor: Eric B Staros, MD more...
Activated clotting time (ACT) depends on the test device used, and it can vary from 80 to 160 seconds. Each manufacturer provides its own baseline reference values and target range with the ACT testing device.
Normal activated clotting time (ACT) indicates that tested blood contains no heparin or that all heparin is inhibited by protamine (postoperative anticoagulation reversal).
ACT is intended to monitor anticoagulant effect of unfractionated heparin. Target ACT values may depend on the specific device and clinical scenario. ACT prolongation also can indicate coagulation factor deficiency, severe thrombocytopenia, or severe platelet dysfunction.
Collection and Panels
Specimen: Whole blood
Collection: Blood (usually 0.5-1 mL) from venous/arterial vessel; indwelling or extracorporeal line is collected into the plastic syringe/tube (see image below) and immediately placed/poured into the device cuvette/cartridge
Storage: Whole blood specimen should be processed within 1 minute (or 2 min if specimen contains therapeutic level of unfractionated heparin)
Activated clotting time (ACT) is a point-of-care coagulation test designed to monitor heparin therapy in the clinical situations in which intensive anticoagulation is required. Similar to partial thromboplastin time (PTT), ACT reflects time of clot formation via the intrinsic coagulation pathway by the addition of factor XII activators (eg, diatomaceous earth [Celite], kaolin, glass beads, ellagic acid) and increases linearly to relation to the heparin concentration. Clotting times may also vary between ACT analyzers manufactured by different (or the same) vendors, depending on the source and the formula of the activator, the amount of activator relative to the sample volume, or the method of clot detection. Therefore, instrument-specific protocols should be established and validated for each type of clinical procedure.
ACT is used for bedside or intraoperative monitoring of unfractionated heparin therapy in the settings of invasive or operative procedures, as follows:
Cardiac catheterization and angiography
Intra-aortic balloon pumping
Percutaneous coronary intervention (PCI)
Extracorporeal membrane oxygenation (ECMO)
Cardiopulmonary bypass graft surgery
The following factors may affect ACT measurements:
Hypothermia (in certain ACT devices)
Drugs: Warfarin, aprotinin, GPIIb/IIIa inhibitors (eg, abciximab)
Severe (< 20 X 10 9/L) but not moderate (40-60 X 10 9/L) thrombocytopenia 
Severe deficiency of contact activation factors (factor XII, prekallikrein, high-molecular-weight kininogen) does not cause increased bleeding risk during heparin therapy but does prolong the ACT and makes it impossible to use this test. Alternative coagulation tests (eg, anti-Xa assay) might be used for heparin monitoring in this situation.[4, 5, 6]
Lupus anticoagulant (LA) also prolongs initial ACT values, and other strategies could be considered for heparin monitoring in patients with LA, such as doubling baseline ACT or in vitro heparin-ACT curves development. Alternatively, the anti-Xa assay or protamine titration devices might be used.
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