Warfarin, a Urinary Infection and a Shop-Bought Pain Tablet
Mr Kamau, 74 years old, 68 kg, has atrial fibrillation and takes warfarin 4 mg daily (target INR 2.0–3.0; last INR 2.6 one week ago), bisoprolol, furosemide and omeprazole. He attends the outpatient clinic with five days of dysuria and frequency. Urine dipstick is positive for nitrite and leucocytes; there is no laboratory culture service on site. The clinical officer prescribes co-trimoxazole 960 mg twice daily for five days and ibuprofen 400 mg three times daily for knee pain. Mr Kamau mentions that he also buys 'pain tablets' from a shop when his knee is bad. He lives alone, 40 km from the clinic, and his INR checks have been three weeks apart. He tells you he feels well and sees no reason to change anything.
1. Identify the two highest-risk aspects of this prescription, explain the mechanism of each, and propose a safer alternative for each with your reasoning.
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Risk 1: co-trimoxazole with warfarin. Sulfamethoxazole inhibits CYP2C9, the main enzyme clearing S-warfarin, and also displaces warfarin from albumin. The INR can climb within three to five days, exactly when he has finished the course and stopped attending. Age, intercurrent infection and reduced oral intake add to the risk. Safer alternative: nitrofurantoin 100 mg twice daily for five days, provided renal function is acceptable (avoid if eGFR is below 30), or fosfomycin 3 g as a single dose. Both have minimal effect on INR. A beta-lactam such as amoxicillin or cephalexin is a further option if local resistance allows. Reasoning: the infection still needs treating; the aim is to choose an agent that does not sabotage anticoagulation.
Risk 2: ibuprofen with warfarin. The NSAID blocks platelet cyclo-oxygenase, impairing platelet function, and directly injures gastric mucosa; there is also protein-binding competition and some CYP2C9 competition. In a 74-year-old also taking furosemide, the combination adds volume depletion and acute kidney injury. The net effect is gastrointestinal bleeding. Omeprazole does not make this safe. Safer alternative: paracetamol, capped at 3 g/day in an older adult, with topical diclofenac gel and non-drug measures (weight loss, quadriceps strengthening, walking stick). Note that regular paracetamol above 2 g/day for several days can raise the INR, so it is not interaction-free - it is simply far safer, with monitoring.
Also address the shop-bought tablets by name; 'pain tablets' in this context are frequently ibuprofen or aspirin-containing combinations.
2. What monitoring and education plan do you put in place before he leaves the clinic?
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Monitoring.
- INR three to five days after starting the antibiotic, then again about a week after finishing it. The peak interaction is delayed; a single check on the day he collects the tablets proves nothing.
- If any bleeding appears, he should have an INR immediately rather than wait for the appointment.
- Basic renal function before nitrofurantoin, if available, and review of furosemide dosing while he is unwell.
- Weight and blood pressure at the next visit; ask about dizziness and postural symptoms.
Education, checked by teach-back rather than delivered as a lecture:
- Why warfarin is not something to stop when he feels well - it prevents stroke, and its absence of symptoms is the point.
- Bleeding warning signs: black tarry stools, blood in urine (already confounded by his dysuria, so he must report any change), unusual bruising, nosebleeds that do not stop, severe headache, vomiting blood.
- No over-the-counter ibuprofen, aspirin or 'pain tablets'; show him the packaging of what he buys.
- Paracetamol limit of 3 g in 24 hours, and not with other paracetamol-containing products.
- Alcohol, consistent green vegetable intake, and no crash diets.
- Sick-day rule: vomiting, diarrhoea or being unable to eat means phoning the clinic, not simply continuing everything.
- Written information with a phone number, and a practical plan for the 40 km journey given that his INR checks have already been three weeks apart - for example, a scheduled date before he leaves and a relative or community health worker reminder.
Document what was taught, what he repeated back, and what he declined.
3. The clinical officer says: "Co-trimoxazole is what we have in stock - just tell him to watch for bleeding." What do you do?
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Treat this as a professional-accountability problem, not a personality clash. Start by acknowledging the constraint: stock shortages are real, and the patient still needs treatment today. Then put the clinical concern in one line - 'co-trimoxazole plus warfarin has a well-recognised interaction that can push the INR up within days, and he lives 40 km away and is checked every three weeks' - and offer concrete options: check whether nitrofurantoin or fosfomycin is on the shelf or at the neighbouring facility, ask the pharmacist, or agree an INR check at day three to five with a clear plan for what to do if it is high.
If the answer does not change: document the discussion, the advice received and the fact that you raised it. Escalate to the nursing officer in charge and, if needed, the medical superintendent or pharmacist in charge. Documenting is not disloyalty; it is the record that both of you were dealing with a stock problem.
With the patient, be honest without undermining his clinician: explain that this antibiotic can make his warfarin stronger, which is why the blood test in three to five days is essential, and what to do if bleeding appears. Record that the risk was discussed and that he understood.
Do not quietly swap the drug yourself or simply reassure him that 'it should be fine'. Both protect the moment and harm the patient.