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Coagulation
 
 

Activated Partial Thromboplastin Time (APTT)

 
 
 
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Overview

    • Normal Range

    • 25 - 40 sec
  • Important Lab Points

  • Collected in a sodium citrate tube to prevent coagulation prior to testing.
  • Performed by contact activation - methods vary between labs.
  • Tests the intrinsic and common pathways - kininogen, prekallikrein, XII, XI, IX, VIII, X, V, II, fibrinogen
  • Practical Points

  • APTT can be used to monitor patients on heparin infusions.
  • APTT does not correspond to the coagulative status of patients on novel anticoagulants such as dabigatran, rivaroxaban and apixaban.
  • APTT does not correspond to the coagulative status of patients with liver disease, and most often these patients are also coagulopathic.

Prolonged APTT

  • Causes of Prolonged APTT

  • Isolated Prolonged APTT

  • Unfractionated heparin - therapeutic or contamination
  • Overcoagulation with low molecular weight heparin
  • Factor deficiency - VIII, IX, VI, XII
  • Factor inhibitors
  • Von Willebrand disease
  • Lupus anticoagulant (antiphospholipid antibody)
  • Prolonged APTT and PT/INR

  • Artefactual - high haematocrit
  • Liver disease
  • Disseminated intravascular coagulation (DIC)
  • Common pathway deficiency - X, V, II, fibrinogen
  • Severe vitamin K deficiency
  • Excess heparin (e.g. from heparin in the line)
  • Warfarin
  • Factor IIa inhibitors (dabigatran)
  • Factor Xa inhibitors (rivaroxaban, apixaban)
  • Approach

  • Determine whether the patient is on heparin or any other anticoagulants.
  • Check the patient’s haematocrit, which if elevated will falsely elevate PT/INR and APTT.
  • Look for evidence of chronic liver disease, either in the patient’s past medical history or current clinical state.
  • Assess the patient’s risk of DIC given their clinical state.
  • If unsure about the cause of prolonged APTT, mixing studies can be performed.

    APTT Mixing Studies

    The patient's plasma is mixed 50:50 with control plasma, and then the APTT is measured.
    • Significance

    • APTT mixing studies to determine the cause of prolonged APTT.
    • Interpretation

    • APTT corrects - factor deficiencyCongenital factor deficiency, Von Willebrand disease, DIC, liver failure, supratherapeutic warfarin
    • APTT does not correct - inhibitor presentHeparin, factor IIa inhibitors, lupus anticoagulant, specific factor inhibitors

    APTT in Heparin Monitoring

    The APTT is used to monitor the coagulative state of patients on heparin infusions, to ensure that they are receiving a therapeutic dose.
    • Practical Points

    • Consult local guidelines for heparin bolusing, rates and titration.
    • Do not take a sample for APTT testing from the same limb through which the patient is recieving heparin. If there is no other option, then stop the heparin infusion, flush the line and wait several minutes before taking the APTT sample.
    • Excess heparin in the line, e.g. in lines that were heparin locked, will prolong both the PT/INR and APTT.
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