Warfarin and a New Antibiotic in a Frail Older Adult
Mrs. R is 84 years old, weighs 48 kg, and lives alone. Her medications are warfarin 3 mg daily (INR stable at 2.4-2.7 for six months), metoprolol succinate 50 mg daily, furosemide 20 mg daily, and omeprazole 20 mg daily. She is diagnosed with a urinary tract infection and the covering clinician starts trimethoprim-sulfamethoxazole (TMP-SMX) double strength twice daily for seven days. Two days later she calls reporting gum bleeding when brushing her teeth and new bruising on both forearms. Her most recent INR, drawn today, is 5.8. She denies melena, hematuria, and headache. She has no caregiver and drives herself to appointments.
1. Explain the interaction and identify which elements of her regimen and clinical picture make this bleed most likely. What immediate action do you recommend?
Read the worked answer
TMP-SMX is a potent inhibitor of CYP2C9, the main enzyme clearing S-warfarin, and it also displaces warfarin from albumin; the combination can raise INR within 3-5 days. Both effects are amplified here: low body weight (48 kg) means a smaller warfarin distribution volume, low albumin is common in frail older adults, and she is already on omeprazole, which can inhibit CYP2C19 and modestly affect warfarin metabolism. Her INR of 5.8 with gum bleeding and bruising is clinically significant bleeding risk even without major hemorrhage. Immediate steps: hold warfarin now, notify the prescriber, and arrange same-day clinical assessment. Because she has active minor bleeding and INR above 4.5, consider oral vitamin K 1-2.5 mg (avoid large doses in a patient without major bleeding to prevent overcorrection and later warfarin resistance). Do not give vitamin K if major bleeding is suspected; then use 4-factor PCC. Recheck INR in 24 hours. Do not simply reduce the warfarin dose and continue TMP-SMX; the interaction persists and INR will keep climbing. Safer antibiotic alternatives include nitrofurantoin if renal function permits, or fosfomycin single dose, depending on culture and CrCl.
2. The prescriber wants to continue TMP-SMX because culture results support it. Describe a monitoring and communication plan that accounts for her living situation.
Read the worked answer
If TMP-SMX must continue, treat it as a high-risk co-prescription. Stop warfarin for 1-2 days until INR is below 3, then restart at a reduced dose (often 20-30% lower) with INR checks at 48-72 hours and again at day 5-7. Because she lives alone and drives, in-person INR monitoring is safer than relying on self-reporting. Arrange a family or neighbor check-in, a pill organizer, and a written bleeding warning list (black stools, pink urine, severe headache, nosebleeds that do not stop). Document the interaction in the chart and on a wallet card. Coordinate with the anticoagulation clinic so the next INR is scheduled before the antibiotic course ends, then again 3-5 days after stopping TMP-SMX, because INR can fall as the inhibition resolves. Avoid adding NSAIDs or aspirin. Educate on fall precautions and soft toothbrush use. This plan addresses both pharmacologic risk and the social determinant of living alone, which the exam expects you to identify.