title: Rifampicin-warfarin interaction leading to macroscopic hematuria: a case report and review of the literature.
abstract: BACKGROUND
  Rifampicin remains one of the first-line drugs used in tuberculosis therapy. This drug's potential to induce the hepatic cytochrome P450 oxidative enzyme system increases the risk of drug-drug interactions. Thus, although the presence of comorbidities typically necessitates the use of multiple drugs, the co-administration of rifampicin and warfarin may lead to adverse drug events. We report a bleeding episode after termination of the co-administration of rifampicin and warfarin and detail the challenges related to international normalized ratio (INR) monitoring.
  
  
  METHODS
  A 59-year-old Brazilian woman chronically treated with warfarin for atrial fibrillation (therapeutic INR range: 2.0-3.0) was referred to a multidisciplinary anticoagulation clinic at a university hospital. She showed anticoagulation resistance at the beginning of rifampicin therapy, as demonstrated by repeated subtherapeutic INR values. Three months of sequential increases in the warfarin dosage were necessary to reach a therapeutic INR, and frequent visits to the anticoagulation clinic were needed to educate the patient about her pharmacotherapy and to perform the warfarin dosage adjustments. The warfarin dosage also had to be doubled at the beginning of rifampicin therapy. However, four weeks after rifampicin discontinuation, an excessively high INR was observed (7.22), with three-day macroscopic hematuria and the need for an immediate reduction in the warfarin dosage. A therapeutic and stable INR was eventually attained at 50% of the warfarin dosage used by the patient during tuberculosis therapy.
  
  
  CONCLUSIONS
  The present case exemplifies the influence of rifampicin therapy on warfarin dosage requirements and the increased risk of bleeding after rifampicin discontinuation. Additionally, this case highlights the need for warfarin weekly monitoring after stopping rifampicin until the maintenance dose of warfarin has decreased to the amount administered before rifampicin use. In particular, patients with cardiovascular diseases and active tuberculosis represent a group with a substantial risk of drug-drug interactions. Learning how to predict and monitor drug-drug interactions may help reduce the incidence of clinically significant adverse drug events.
authors: Faculdade de Farmácia, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil. auxiliadorapmartins@hotmail.com
authors: Maria A P
authors: MA
authors: Martins
authors: Adriano M M
authors: AM
authors: Reis
authors: Mariana F
authors: MF
authors: Sales
authors: Vandack
authors: V
authors: Nobre
authors: Daniel D
authors: DD
authors: Ribeiro
authors: Manoel O C
authors: MO
authors: Rocha
authors: Antônio L P
authors: AL
authors: Ribeiro
mesh_terms: D000904:Antibiotics, Antitubercular / Q000008:administration & dosage / Q000009:adverse effects*; D000925:Anticoagulants / Q000008:administration & dosage / Q000009:adverse effects*; D001281:Atrial Fibrillation / Q000188:drug therapy; D004347:Drug Interactions; D005260:Female; D006417:Hematuria / Q000097:blood / Q000139:chemically induced*; D006801:Humans; D008875:Middle Aged; D012293:Rifampin / Q000008:administration & dosage / Q000009:adverse effects*; D014397:Tuberculosis, Pulmonary / Q000188:drug therapy; D014859:Warfarin / Q000008:administration & dosage / Q000009:adverse effects*
chemical_list: D000904:Antibiotics, Antitubercular; D000925:Anticoagulants; D014859:Warfarin; D012293:Rifampin
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