## Clinical Context The patient has developed a supratherapeutic INR (4.8) after starting amoxicillin-clavulanate. Beta-lactam antibiotics inhibit warfarin metabolism and displace it from plasma protein binding, increasing anticoagulant effect. ## Mechanism of Interaction **Key Point:** Amoxicillin-clavulanate is a **moderate inhibitor of CYP2C9**, the primary enzyme metabolizing warfarin's active S-enantiomer. This reduces warfarin clearance and prolongs INR. **High-Yield:** Beta-lactam antibiotics (especially amoxicillin, ampicillin, and fluoroquinolones) are classic warfarin-potentiating drugs. Other notable interactions include NSAIDs, aspirin, and azole antifungals. ## Management Algorithm ```mermaid flowchart TD A["Elevated INR on warfarin + drug interaction"]:::outcome --> B{"INR level & bleeding?"}:::decision B -->|"INR 4.5–10, no bleeding"| C["Reduce warfarin 20–30%"]:::action C --> D["Recheck INR in 3–5 days"]:::action B -->|"INR > 10 or minor bleeding"| E["Hold warfarin + vitamin K 2.5–5 mg PO"]:::action B -->|"Major bleeding"| F["Vitamin K 10 mg IV + FFP/PCC"]:::urgent D --> G["Adjust further if needed"]:::action E --> H["Recheck INR 24 hrs later"]:::action ``` ## Rationale for Correct Answer 1. **INR 4.8 without bleeding** → dose reduction (not cessation) is appropriate. 2. **20–30% reduction** in warfarin dose is standard for this INR range. 3. **Recheck in 3–5 days** allows time for the new dose to reach steady state and for the antibiotic interaction to stabilize. 4. **Vitamin K is NOT routinely given** unless INR >6 or there is bleeding. **Clinical Pearl:** Once the infection resolves and the antibiotic is stopped, warfarin metabolism will normalize and INR may drop — the dose may need to be increased again. **Tip:** Always counsel patients on warfarin to report new medications and infections; maintain consistent vitamin K intake.
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