Activated Clotting Time

Updated: Nov 27, 2019
Author: Vadim Kostousov, MD; Chief Editor: Eric B Staros, MD 

Reference Range

The normal range for ACT is 70-120 sec, with the therapeutic range for anticoagulation being 150-600 sec. (However, these ranges vary according to the test device used and the therapy employed.)[1]



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)

Glass vacuum tube. Glass vacuum tube.



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.[2] 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.[3]


ACT is used for bedside or intraoperative monitoring of unfractionated heparin therapy in the settings of invasive or operative procedures, as follows:

  • Dialysis

  • Cardiac catheterization and angiography

  • Intra-aortic balloon pumping

  • Percutaneous coronary intervention (PCI)

  • Extracorporeal membrane oxygenation (ECMO)

  • Vascular surgery

  • Valve replacements

  • Carotid endarterectomy

  • Cardiopulmonary bypass graft surgery


The following factors may affect ACT measurements:

  • Hemodilution

  • Hypothermia (in certain ACT devices)

  • Drugs: Warfarin, aprotinin, GPIIb/IIIa inhibitors (eg, abciximab)

  • Severe (< 20 X 109/L) but not moderate (40-60 X 109/L) thrombocytopenia[4]

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.[5, 6, 7]

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.[8]