Melaena and INR 8.5 in a 76-year-old woman on warfarin after starting clarithromycin

Case presentation: A 76-year-old woman was brought to the emergency department by her daughter with a one-day history of dizziness and black, tarry stools. She has a background of atrial fibrillation, hypertension, type 2 diabetes mellitus, and hypercholesterolaemia. Her regular medications, all long-term and at stable doses, are warfarin 5 mg once daily (target INR 2.0-3.0), lisinopril 10 mg once daily, metoprolol 50 mg twice daily, metformin 500 mg twice daily, and atorvastatin 20 mg once daily at night. A recent INR one month ago was 2.4.

Four days before admission, she presented to her GP with a three-day history of fever, productive cough, and right-sided pleuritic chest pain. Community-acquired pneumonia was diagnosed, and she was started on clarithromycin 500 mg twice daily. She has continued her regular medicines. Her appetite has been poor since the onset of the illness. She has not had any change in alcohol consumption, and she has not taken any new over-the-counter or herbal preparations.

On arrival in the emergency department, she appears pale and clammy. Her heart rate is 112 beats per minute, blood pressure 92/58 mmHg, temperature 37.1°C, respiratory rate 18 breaths per minute, and oxygen saturation 96% on room air. Abdominal examination is soft and non-tender, and digital rectal examination reveals tarry, black stool. Cardiovascular and respiratory examinations are otherwise unremarkable.

Investigations: INR 8.5 (previous INR 2.4), haemoglobin 8.0 g/dL (previous 12.5 g/dL), platelet count 215 × 10^9/L. Urea, creatinine, electrolytes, liver proteins and transaminases are all within the reference range. Chest X-ray confirmed right lower lobe consolidation.

She was admitted to the acute medical unit and treated with intravenous vitamin K 10 mg and prothrombin complex concentrate. Warfarin was withheld. Clarithromycin was discontinued, and she was switched to doxycycline 100 mg twice daily for the pneumonia. She received two units of packed red cells. Over the next 24 hours, her INR fell to 2.1, her haemoglobin stabilised at 9.5 g/dL, and the melaena resolved. She was discharged with a plan to restart warfarin at a lower dose under the anticoagulation clinic. There was no rechallenge with clarithromycin.