What are considerations when using activated partial thromboplastin time (aPTT) to monitor anticoagulant therapy?

Updated: Jul 30, 2019
  • Author: Muhammad Bader Hammami, MD; Chief Editor: Eric B Staros, MD  more...
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Answer

Answer

Because factors II, IX, and X are vitamin K–dependent, biliary obstruction, which precludes gastrointestinal absorption of fat and fat-soluble vitamins (including vitamin K), can reduce their concentrations and thus prolong the aPTT. [1]

aPTT is used to monitor heparin anticoagulant therapy; however, it cannot be used to monitor therapy with newer low-molecular-weight heparin. [1, 2]

Prolonged aPTT is usually followed by mixing studies (when the cause is not obvious, eg, due to heparin contamination or to other preanalytical problems such as an insufficient or clotted blood sample) to evaluate for deficient coagulation factor(s) or a coagulation inhibitor(s). [10]

In mixing studies, the patient’s plasma is mixed with normal plasma. If mixing the two plasma samples corrects the aPTT result, there is clotting factor deficiency, and specific coagulation factor testing is performed to determine which factor(s) is deficient. If the mixing fails to correct the aPTT results within 3-4 seconds, it strongly suggests (1) a coagulation factor inhibitor (eg, an acquired factor VIII antibody) or (2) an antiphospholipid antibody or lupus anticoagulant (a nonspecific inhibitor). In this case, the aPTT result will not correct with normal plasma mixing but it will usually correct if an excess of phospholipid is added to the sample. If lupus anticoagulant is suspected, a more sensitive test, lupus anticoagulant–sensitive aPTT or Dilute Russell Viper Venom Test, should be performed. [2]


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