Most anticoagulant therapies need ongoing or selective testing for assessment of therapeutic effect. Appropriate monitoring ensures that these agents are maintained within the therapeutic range; otherwise patients are at risk of thromboembolism and bleeding complications.
- Warfarin Anticoagulation. Warfarin therapy must be monitored by the PT and its analogue for this purpose, the international normalized ratio (INR), to avoid under- or overcoagulation.
- PT methods and reagents can widely differ between laboratories, yielding varying PT values for the same degree of factor deficiency.
- The INR is a normalized value that is intended to compare results across laboratories
- Heparin Anticoagulation Testing. The aPTT is used to assess heparin anticoagulation. Each laboratory determines its own therapeutic target range for heparin anticoagulation, typically on the order of 1.5 to 2.5 times the normal mean.
- Low-molecular-weight heparin (LMWH) drugs and their analogue, synthetic pentasaccharide (fondaparinux), do not affect the aPTT assay, and coagulation testing is usually not needed.
- Heparin and to a lesser degree LMWH can stimulate the production of antibodies against the heparin-platelet factor 4 complex. These antibodies can in turn cause heparin-induced thrombocytopenia (HIT) or activation of platelets to induce thrombosis.
- Newer anticoagulants (alternatives to heparin in patients with HIT and to patients being treated with warfarin) are direct anticoagulants (rivaroxaban, apixaban) that are not mediated by antithrombin. They prolong the PT and aPTT, but monitoring is not recommended, and the INR should not be used.
- Direct thrombin inhibitors (DTIs) prolong the PT and aPTT and interfere with clot-based fibrinogen assays.