Monitor for side effects at lower therapeutic ranges

Geri: Patients over 60 yr exhibit greater than expected PT/INR response.

Asian patients and those who carry the CYP2C9*2 allele and/or the CYP2C9*3 allele, or those with VKORC1 AA genotype may require more frequent monitoring and lower doses. Heparin may affect the PT/INR draw blood for PT/INR in patients receiving both heparin and warfarin at least 5 hr after the IV bolus dose, 4 hr after cessation of IV infusion, or 24 hr after subcut heparin injection. Lower levels are acceptable when risk is lower. An INR of 2.5–3.5 is recommended for patients at very high risk of embolization (for example, patients with mitral valve replacement and ventricular hypertrophy). Normal INR (not on anticoagulants) is 0.8–1.2. Therapeutic PT ranges 1.3–1.5 times greater than control however, the INR, a standardized system that provides a common basis for communicating and interpreting PT results, is usually referenced. Monitor PT, INR, and other clotting factors frequently during therapy monitor more frequently in patients with renal impairment.

Assess for evidence of additional or increased thrombosis.

Assess for signs of bleeding and hemorrhage (bleeding gums nosebleed unusual bruising tarry, black stools hematuria fall in hematocrit or BP guaiac-positive stools, urine, or nasogastric aspirate).

Ingestion of large quantities of foods high in vitamin K content (see list in food sources for specific nutrients) may antagonize the anticoagulant effect of warfarin.

↑ bleeding risk with anise, arnica, chamomile, clove, dong quai, fenugreek, feverfew, garlic, ginger, ginkgo, Panax ginseng, licorice , and others.Many other drugs may affect the activity of warfarin.Barbiturates, carbamazepine, rifampin, and hormonal contraceptives containing estrogen may ↓ the anticoagulant response to warfarin.

Acute alcohol ingestion may ↑ action of warfarin.Chronic alcohol ingestion may ↓ action of warfarin if chronic alcohol abuse results in significant liver damage, action of warfarin may be ↑ due to ↓ production of clotting factor.

Chronic use of acetaminophen may ↑ the risk of bleeding.

Androgens, capecitabine, cefotetan, chloramphenicol, clopidogrel, disulfiram, fluconazole, fluoroquinolones, itraconazole, metronidazole (including vaginal use), thrombolytics, eptifibatide, tirofiban, sulfonamides, quinidine, quinine, NSAIDs, valproates, and aspirin may ↑ the response to warfarin and ↑ the risk of bleeding.

Warfarin overdose antidote