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Lab Test: Activated Clotting Time (ACT; Activated Coagulation Time)

    Lab Names
    • Activated Clotting Time; Ground Glass Clotting Time; and Whole Blood Activated Clotting Time
    • Other names:
      • Ground glass clotting time
      •  Whole blood activated clotting time
    Description
    • Measures the time for whole blood to clot after the addition of particulate activators, which first activates factor XII and measures the intrinsic pathway's ability to begin clot formation.  By checking the blood clotting status with ACT, the response to unfractionated heparin therapy can be easily and rapidly monitored. 
    • It is primarily used to measure the anticoagulant effect of unfractionated heparin (UFH) or other direct thrombin inhibitors (DTI) during cardiac angioplasty, hemodialysis, and cardiopulmonary bypass graft (CABG) surgery. 
    • It tends to be more accurate than the aPTT when high doses of heparin are used for anticoagulation. 
    • It is less expensive and more easily and rapidly performed than the aPTT providing immediate information on to titrate the medications for the clinical situation.
    Reference Range
    • Normal = 70-120 seconds (some assays will say up to 180 seconds)
    • Therapeutic range for anticoagulation =  150-600 seconds & varies with indication
      • Note: This will vary according to the indication - see below
    Indications
    • Hemodialysis:  therapeutic range of 132 to 234 seconds.  It can also help guide the use of protamine sulfate needed to reverse the effect.
    • PTCA:  HemoTec > 275 to 300 seconds, or Hemochron > 340 to 400 seconds.  
    • CABG:  The accepted goal for the ACT is 400-480 seconds.
    • Vascular catheterization, hemodialysis, or extracorporeal membrane oxygenation (ECMO):  determine the heparin dose from a standard nomogram adjusted for the patient's baseline ACT.
    • The modified ACT test requires a smaller-volume blood specimen, automated blood sampling, standardized blood/reagent mixing and faster clotting times results that the conventional ACT.
    Clinical Application

    For increased levels:

    • Patients receiving heparin which binds to antithrombin to inhibit clotting factors mainly of the intrinsic pathway. 
    • Patients receiving warfarin (Coumadin) which reduces the functional activation of vitamin K dependent clotting factors
    • Cirrhosis which results in decreased production of clotting factors
    • A deficiency in clotting factors.
    • Lupus anticoagulants against components involved in the activation of the coagulation cascade

    Assess the patient to detect possible bleeding including blood in the urine and the presence of bruises, petechiae, and/or low back pain.


    For decreased levels:

    • Activate of the thrombotic pathways can shorten the ACT.
    Related Tests
    • Partial Thromboplastin Time (PTT)
    • Prothrombin Time
    • Coagulating Factor Concentration
    Lab Interactions

    The ACT can be affected by several biologic variables, including:  

    • Hypothermia
    • Hemodilution
    • Platelet number & functions
    • Factors affecting the pharmacokinetics of heparin (e.g., kidney or liver disease)
    • Antithrombin (AT) deficiency
    • A partially or completely occluded specimen can increase ACT measurements
    Test Tube
    • Black top tube containing a particulate activator
    Lab Procedure
    • If the patient is receiving a continuous heparin drip, the blood sample is obtained from the arm without the intravenous catheter
    • Less than 1 ml of blood is collected into a commercial container. 
    • The container is placed into a whole blood micro-coagulation analyzer. 
    • When a clot has formed, the ACT value is displayed on the machine's panel.
    • Apply pressure to the venipuncture site after procedure.  Remember that the bleeding time will be prolonged because of anticoagulation therapy. 
    • For clinical significance, the test results must be correlated with the time of the heparin administration.  A clinical flow sheet is used to list the test results with the time and route of heparin administration.
    Storage & Handling
    • The sample should not be stored.  It should be tested immediately after it has been obtained.
    Patient Counseling
    • No preparation needed by the patient.
    References
    • LaGow B et al., eds. PDR Lab Advisor. A Comprehensive Point-of-Care Guide for Over 600 Lab Tests.  First ed. Montvale, NJ: Thomson PDR; 2007.
    • Pagana K, Pagana TJ eds. Mosby's Manual of Diagnostic and Laboratory Tests. 5th Ed.  St. Louis, Missouri. 2014.
    • Olson JD et al. College of American Pathologists Conference XXXI on Laboratory Monitoring of Anticoagulant Therapy.  Laboratory monitoring of unfractionated heparin therapy.  Arch Pathol Lab Med 1998;122(9):782-98.  PMID:  9740136