Warfarin

# Warfarin ## Overview - **Classification**: Anticoagulant, Vitamin K Antagonist (VKA). - **Mechanism**: Inhibits Vitamin K epoxide reductase (VKORC1), depleting active vitamin K. This impairs synthesis of clotting factors II, VII, IX, X, and proteins C and S. ## Primary Indications 1. **Deep Vein Thrombosis (DVT) & Pulmonary Embolism (PE)**: Prevention and treatment. 2. **Atrial Fibrillation (AF)**: Prevention of thromboembolism (stroke). 3. **Mechanical Heart Valve Replacement**: Prevention of thromboembolism. 4. **Recurrent Transient Ischemic Attacks (TIAs) / Stroke**: Prevention of systemic embolism. ## Adult Dosing ### Standard Dosing **Initial Dosing (General)** - **Dose**: **2 mg to 5 mg** once daily. - **Frequency**: Once daily. - **Route**: Oral. - **Goal**: Achieve target **INR 2.0-3.0**. (INR 2.5-3.5 for mechanical mitral valves or high risk). - **Duration**: Varies by indication (e.g., DVT/PE typically 3-6 months, AF long-term). - **Considerations**: Use **2.5 mg** initial dose for elderly, debilitated, hepatic impairment, or heart failure. - **Considerations**: Also for high bleeding risk or high sensitivity (e.g., CYP2C9\*2/\*3 or VKORC1 AA genotype). **Maintenance Dosing** - **Dose**: Individualized based on **INR** response. - **Frequency**: Once daily. - **Route**: Oral. - **Adjustment**: Titrate to maintain target INR. ### Dose Adjustments - **Renal Impairment**: No specific dose adjustment. Monitor INR closely due to increased bleeding risk in ESRD. - **Hepatic Impairment**: Start with lower doses (**≤2.5 mg**). Monitor INR closely due to impaired clotting factor synthesis. - **Elderly Patients**: Start with lower doses (**≤2.5 mg**) due to increased sensitivity and bleeding risk. Monitor INR more frequently. ## Pediatric Dosing *Note: Pediatric use requires close monitoring and expert consultation due to wide interpatient variability and increased risk of bleeding or subtherapeutic INR.* ### General Pediatric Considerations (0-18 years) - **Goal INR**: **2.0-3.0**. - **Bridging**: Initiate therapy only after adequate parenteral heparin or LMWH for at least 5 days. - **Monitoring**: Close INR monitoring by experts is essential due to high variability. ### Neonates (0-28 days) - **Initial Dose**: **0.1 mg/kg** once daily for 2 doses (if liver function is normal). - **Frequency**: Once daily. - **Maximum**: **2.5 mg** initial dose. - **Maintenance**: **0.05-0.34 mg/kg/day**. - **Special Notes**: Highly variable response. Requires very close INR monitoring. - **Special Notes**: Avoid until heparin/LMWH bridging is established due to low Vitamin K stores. ### Infants (1-12 months) - **Initial Dose**: **0.2 mg/kg** once daily for 2 doses. - **Maximum**: **5 mg** initial dose. - **Maintenance**: **0.05-0.34 mg/kg/day**. - **Special Notes**: High variability. Close INR monitoring essential. - **Special Notes**: Consider lower initial dose for those at increased risk of bleeding. ### Children (1-12 years) - **Initial Dose**: **0.2 mg/kg** once daily for 2 doses. - **Maximum**: **10 mg** initial dose. - **Maintenance**: **0.05-0.34 mg/kg/day** (typically **3-10 mg/day**). - **Special Notes**: Children often require higher mg/kg doses than adults due to faster metabolism. ### Adolescents (13-18 years) - **Dose**: Approach adult dosing, typically starting **2.5-5 mg** once daily. - **Maximum**: Adult maintenance dose (up to **10 mg/day**). - **Special Notes**: Consider lower initial doses in smaller adolescents or those with risk factors. ## Safety Information ### Contraindications - **Absolute**: Pregnancy (teratogenic, fetal hemorrhage). - **Absolute**: Uncontrolled severe hypertension. - **Absolute**: Recent or planned neurosurgery/ophthalmic surgery. - **Absolute**: Aneurysm, cerebrovascular hemorrhage. - **Absolute**: Active major bleeding (GI, GU, respiratory tract, CNS). - **Absolute**: Thrombocytopenia, known bleeding diathesis. - **Absolute**: Lumbar puncture, spinal anesthesia (within 24 hrs prior/1 hr after). - **Relative**: History of falls, dementia, advanced liver/kidney disease. ### Common Adverse Effects - **Very Common (>10%)**: Bleeding (minor: bruising, epistaxis, gum bleeding). - **Common (1-10%)**: Nausea, diarrhea, abdominal pain, flatulence. - **Serious but Rare**: Major hemorrhage (intracranial, GI), skin necrosis, purple toe syndrome, hepatotoxicity. ### Key Drug Interactions - **Aspirin/NSAIDs**: Increased bleeding risk; caution or avoid concurrent use. - **Clopidogrel/Other Antiplatelets**: Significantly increased bleeding risk. - **Antibiotics (e.g., Metronidazole, Bactrim, Fluconazole)**: Potentiate warfarin, increase INR. Monitor INR closely, reduce warfarin dose. - **Amiodarone**: Potentiates warfarin, significant INR increase. Reduce warfarin dose by 30-50% when starting amiodarone. - **CYP2C9/3A4 Inducers (e.g., Rifampin, Carbamazepine, Phenobarbital)**: Decrease warfarin effect, reduce INR. Increase warfarin dose. - **Foods High in Vitamin K (e.g., leafy green vegetables)**: Decrease warfarin effect. Maintain consistent intake, do not avoid. ## Monitoring & Follow-up - **Before Treatment**: Baseline INR, CBC with platelets, LFTs, renal function. Pregnancy test for women of childbearing potential. - **During Treatment**: - **INR**: Daily until stable for 2 consecutive days. Then 2-3 times/week for 1-2 weeks. Then weekly, then every 2-4 weeks. More frequent with dose changes or new interacting drugs. - **Clinical Signs**: Monitor for signs of bleeding (bruising, melena, hematuria, epistaxis). - **Clinical Signs**: Regular assessment for efficacy (absence of thrombotic events) and safety (absence of bleeding). ## Clinical Pearls - 💡 **Consistent Intake**: Take warfarin at the **same time daily** to maintain stable drug levels. - 💡 **Vitamin K Consistency**: Advise patients to maintain a **consistent dietary intake of Vitamin K-rich foods** rather than avoiding them. - 💡 **INR Target**: Clearly communicate **target INR range** to the patient and reason for therapy. - 💡 **Bridging Therapy**: Warfarin takes several days to achieve full anticoagulant effect; typically **bridge with heparin/LMWH** for at least 5 days and until INR is therapeutic for 2 consecutive days. - 💡 **Antidote**: **Vitamin K** is the primary antidote for warfarin overdose. For rapid reversal, use **4-factor PCC**. - 💡 **Patient Education**: Emphasize importance of **reporting all new medications** (OTC, herbal) and **any signs of bleeding** to healthcare provider immediately. > **⚠️ Important**: This information is for educational purposes only. Always consult current prescribing information, local guidelines, and clinical judgment before prescribing.