A client is receiving warfarin (Coumadin). What lab value is monitored to assess effectiveness?

Warfarin is monitored using PT/INR. Therapeutic INR ranges: 2.0-3.0 for most conditions (atrial fibrillation, DVT, PE, after major heart valve surgery); 2.5-3.5 for mechanical heart valves and recurrent thromboembolism. INR above 3.0-3.5 (depending on indication) increases bleeding risk; below the therapeutic range provides inadequate anticoagulation. PTT (or aPTT) monitors heparin and is targeted at 1.5-2.5 times the control. Antidotes: vitamin K reverses warfarin (slowly, hours to days); protamine sulfate reverses heparin (rapidly). Warfarin teaching: consistent intake of vitamin K-containing foods (green leafy vegetables — eat consistent amounts rather than avoiding); avoid alcohol; report unusual bleeding (gums, urine, stool); many drug interactions (especially antibiotics, NSAIDs, amiodarone) — coordinate all medications with prescriber. Direct oral anticoagulants (DOACs like apixaban, rivaroxaban) don't require routine INR monitoring.