A 57-year-old man is staed on heparin therapy after being diagnosed with a small pulmonary embolism. The anticoagulant effect of this medication would be most appropriately assessed by which of the following laboratory values?

Heparin is often administered to individuals diagnosed with acute embolism or thrombosis by continuous IV infusion at a rate sufficient to raise the activated paial thromboplastin time (APTT) to 1.5-2 times the control value. In other words, the anticoagulant effect of heparin should be continually monitored to ensure that the patient's APTT does not exceed 1.5-2 times the control value. If the control APTT value is exceeded, the risk for internal bleeding can substantially increase. The bleeding time is a measure of platelet function and correlates well with the propensity to bleed. A complete blood count is a measure of all the cell components contained in the blood, e.g., the number of white blood cells, red blood cells, and platelets. Both the international normalized ratio (INR) and prothrombin time (PT) are used to monitor the anticoagulant effect of coumarin anticoagulants, such as warfarin. Coumarin anticoagulants are dosed to raise the PT to 1.5-3.5 times the control value. However, the thromboplastins used to monitor PT values vary among institutions, leading to different control values and subsequently different levels of anticoagulation for a given patient. In other words, there is no absolute consistency for a PT value taken at two different hospitals. Therefore, INR values are commonly used because the INR at one institution will equal that at another.