Warfarin, Antibiotic, and Over-the-Counter NSAID
A 64-year-old man with a mechanical mitral valve takes warfarin 5 mg daily; his INR has been stable at 2.5–3.0. Three days ago an urgent care clinic prescribed trimethoprim-sulfamethoxazole for a urinary tract infection. He also began taking naproxen 500 mg twice daily for knee pain because "it's over the counter." Today he reports gum bleeding when brushing and a 6 cm purple bruise on his forearm. Vitals: BP 138/82, HR 92, RR 18, temperature 37.0°C. Labs: INR 5.8, hemoglobin 11.2 g/dL (baseline 13.4), platelets 190,000. He has no melena, hematemesis, or neurologic symptoms. He asks whether he should "just skip tonight's warfarin" and continue the antibiotic.
1. Which findings concern you most, and what is your immediate priority?
Read the worked answer
The most concerning finding is the combination of an INR of 5.8, visible bleeding, a hemoglobin drop of 2.2 g/dL, and two interacting medications. Trimethoprim-sulfamethoxazole inhibits warfarin metabolism and can raise the INR; naproxen adds antiplatelet effect and gastrointestinal bleeding risk. Gum bleeding and a large bruise show that he is already bleeding, even without melena or hematemesis. Neurologic symptoms would raise concern for intracranial bleeding, but their absence does not make him safe.
Immediate priority is to assess for serious bleeding and notify the anticoagulation prescriber or covering clinician now. Check vital signs, mental status, and for hidden bleeding such as back pain, abdominal pain, or dark stools. Hold the next warfarin dose until the prescriber advises, but do not tell him to stop the antibiotic on his own. Repeat INR and hemoglobin per order, and ask whether vitamin K is indicated. Vitamin K is not automatic for every INR above 5; the decision depends on bleeding severity and the prescriber's plan. Avoid intramuscular injections and invasive procedures until the plan is clear.
2. What medication reconciliation and communication steps would you take?
Read the worked answer
Complete a full medication reconciliation: prescription drugs, over-the-counter products, supplements, and adherence. Specifically identify warfarin dose and last dose, trimethoprim-sulfamethoxazole start date and dose, naproxen frequency, and any aspirin, fish oil, or herbal products. Ask about alcohol use and recent falls.
Call the urgent care prescriber and the anticoagulation clinic or primary prescriber. State the INR, hemoglobin, bleeding signs, and the interacting medications. Ask for a specific plan: whether to hold warfarin, whether to give vitamin K, when to recheck INR, and whether an alternative antibiotic is needed. Document the names of the people contacted, the time, the recommendations, and the read-back.
Also speak with the patient about stopping naproxen now and not substituting another NSAID. If he needs pain relief, ask the prescriber about acetaminophen at a safe dose. Send a message to the pharmacy so the interaction is flagged for future fills.
3. What would you teach the patient before he leaves, and how would you confirm understanding?
Read the worked answer
Teach in plain language, focusing on safety actions rather than a lecture on pharmacology:
- "Your INR is high, which means your blood is clotting too slowly. You have gum bleeding and a bruise, so we need to lower that risk."
- Do not take naproxen, ibuprofen, aspirin, or other NSAIDs unless the prescriber specifically approves them. Use acetaminophen only at the recommended dose.
- Do not skip or double warfarin doses on your own; follow the prescriber's exact instructions.
- Call immediately for black or bloody stools, vomiting blood, severe headache, weakness, confusion, or bleeding that will not stop.
- Keep all INR appointments and tell every prescriber and pharmacist that you take warfarin.
Use teach-back: "Tell me in your own words which pain medicine you will avoid and what you will do if you see black stools." Ask him to show you the warfarin bottle and read the dose. Document the teach-back and his response. Provide written instructions at a readable level and confirm a follow-up appointment.