What laboratory test is primarily used to monitor patients on heparin therapy?

Monitoring patients on heparin therapy is crucial to ensure therapeutic efficacy while minimizing the risk of bleeding. The primary laboratory test used for this purpose is the activated partial thromboplastin time (APTT or PTT). This test measures the time it takes for blood to clot and evaluates the intrinsic and common coagulation pathways, which are affected by heparin.

Heparin works by enhancing the activity of antithrombin III, which inhibits thrombin and factor Xa, both of which are part of the coagulation cascade evaluated by the PTT. A prolonged PTT indicates that the anticoagulation effect of heparin is present, guiding healthcare providers to adjust doses accordingly.

In contrast, prothrombin time (PT) and international normalized ratio (INR) are primarily used for monitoring vitamin K antagonists like warfarin, as they assess the extrinsic pathway of coagulation. A complete blood count (CBC) provides valuable information about blood cell levels but does not specifically monitor coagulation status related to heparin therapy. Therefore, PTT is the most relevant test in this context and is essential for safe and effective management of patients receiving heparin.