Continuous unfractionated heparin infusion is used to prevent thrombosis. It carries a high risk for bleeding if serum concentrations are too high, so regular monitoring is required. The most widely accepted monitoring test is activated partial thromboplastin time (aPTT). Anti-Xa is also used. Plasma or whole blood heparin concentrations are not routinely used for this purpose because the amount of heparin is unimportant; it is the heparin effect that must be monitored. International normalized ratio (INR) is commonly used to assess anticoagulation in people taking warfarin. Prothrombin time may be prolonged due to vitamin K antagonists (such as warfarin), vitamin K deficiency, liver disease, and disseminated intravascular coagulation.