Background and importance Fidaxomicin is a macrolide antibiotic used to treat intestinal Clostridium difficile (CD) infection in the absence of metronidazole or vancomycin treatments. Pharmacovigilance collects information, and analyses and notifies cases of suspected adverse drug reactions (ADRs) to prevent them occurring in the future.

Aim and objectives To describe a case of metabolic acidosis in a patient treated with fidaxomicin and establish its possible association.

Material and methods We describe the case of an 82-year-old man diagnosed with multiple myeloma and treated with two full cycles of bortezomib–dexamethasone. He was referred to the emergency department after presenting with melenic diarrhoea for 1 week. As a result, he was hospitalised and diagnosed with upper gastrointestinal bleeding, acute prerenal renal failure, mild thrombopenia, hypokalaemia and hyponatraemia. After fluid and electrolyte stabilisation, it was decided to start fidaxomicin 200 mg/12 hours due to fever, confusional syndrome, persistence of diarrhoea and positive CD toxin test. The following constants were measured to confirm metabolic acidosis: gas level of bicarbonate (HCO₃ ⁻), partial pressure of carbon dioxide (pCO₂), hydrogen ion potential (pH) and anion GAP. The degree of drug adverse reaction causality was evaluated using the Naranjo algorithm.

Results Two blood gas tests on consecutive days confirmed very low HCO₃ ⁻ (9 mmol/L) and pCO₂ (16 mm Hg) with normal pH (7.4), after which the patient was diagnosed with compensated metabolic acidosis with normal GAP anion. Finally, it was decided to suspend fidaxomicin and in the following days the patient experienced a progressive clinical improvement. Naranjo’s algorithm established the causality relationship as ‘probable’ (score of 6). The regional pharmacovigilance centre (RPC) was notified.

Conclusion and relevance The European Medicines Agency’s technical sheet for fidaxomicin does not describe metabolic acidosis as an ADR. However, UpToDate Clinical Library reports <2% of cases of metabolic acidosis in adults treated with fidaxomicin. The RPC reported this case as the only fidaxomicin ADR notified in our country.

Conflict of interest No conflict of interest