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When managing a pulmonary embolism with unfractionated heparin, the effect of the anticoagulant is monitored with the activated partial thromboplastin time (aPTT). Heparin works by enhancing antithrombin III, which slows the intrinsic and common coagulation pathways. The aPTT measures how long blood takes to clot through these pathways, so adjusting heparin dosing to keep the aPTT in a therapeutic range (typically about 1.5 to 2.5 times the normal value, depending on the lab) ensures effective anticoagulation while limiting bleeding risk.

The other values—sodium, creatinine, and ALT—assess electrolytes, kidney function, and liver function, respectively, and do not reflect how anticoagulation is affecting clotting. Hence they’re not used to monitor anticoagulation therapy.