Warfarin Interaction with Antibiotic and Antifungal Therapy
A 68-year-old man with atrial fibrillation is stable on warfarin 5 mg daily (INR 2.5–3.0 for the past 6 months). He presents with a productive cough and fever. He is prescribed amoxicillin-clavulanate 875/125 mg twice daily for 10 days. Three days later, he develops oral thrush and is started on fluconazole 200 mg daily for 7 days. His medications also include metoprolol, lisinopril, and atorvastatin. On day 5 of antibiotics, he reports gum bleeding and dark stools. His INR is 6.8.
1. Identify the factors that contributed to the supratherapeutic INR and explain the mechanism.
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The supratherapeutic INR is likely due to multiple factors:
- Fluconazole is a potent inhibitor of CYP2C9, the primary enzyme metabolizing S-warfarin. This reduces warfarin clearance, increasing its effect.
- Amoxicillin-clavulanate can disrupt gut flora that produce vitamin K, reducing vitamin K availability and enhancing warfarin's anticoagulant effect. Additionally, acute infection and fever can increase warfarin sensitivity.
- Patient factors: age, possible reduced oral intake due to illness, and drug interactions.
The combination of CYP2C9 inhibition and reduced vitamin K synthesis led to a markedly elevated INR and bleeding.
2. Outline the immediate management and monitoring plan for this patient.
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Immediate management:
- Assess bleeding severity: gum bleeding and dark stools suggest possible GI bleeding; check hemoglobin, hematocrit, and vital signs.
- Withhold warfarin and give vitamin K1 (phytonadione) 2.5–5 mg orally (or 1–2 mg IV if active major bleeding) to reverse anticoagulation.
- If major bleeding, consider prothrombin complex concentrate (PCC) or fresh frozen plasma.
- Monitor INR daily until stable.
- Consider alternative antifungal (e.g., nystatin for oral thrush) to avoid fluconazole interaction.
- If antibiotic necessary, choose one with minimal warfarin interaction (e.g., azithromycin) and monitor INR closely.
- Educate patient on bleeding signs and dietary vitamin K consistency.
- Restart warfarin at lower dose when INR <5 and bleeding resolved, with close monitoring.