Warfarin Interactions with Antibiotics in Children;

Warfarin suppresses the formation of coagulation factors by inhibiting vitamin K. This action is enhanced by the antibiotics on the intestinal flora. The intestinal flora of children matures as they grow and the effects of antibiotics on warfarin anticoagulation may change with the growth of children. In this study, an alteration of prothrombin time-international normalized ratio (PT-INR) before and after antibiotic treatment in children younger than 4 years old receiving warfarin was reviewed retrospectively using medical charts of patients from January 2013 to March 2019. Records for 12 patients (median age; 2 years and 3 months) receiving 14 therapeutic courses of antibiotics under warfarin treatment were included for analysis. All of the 12 cases showed PT-INR prolongation after antibiotic administration, and the median (range) rate of change in PT-INR was 1.25 (1.04 to 2.25). Four patients reduced their warfarin dosage due to antibiotic administration. The incident of PT-INR prolongation tended to be higher in younger patients. Other patient backgrounds such as the types of antibiotics, delivery methods and the status of nutrition, which impact on the intestinal flora, were not associated with the PT-INR prolongation by antibiotic treatment. This study was a small case series and further investigation is required to clarify the factors of the change in PT-INR and optimize the warfarin dosage for children during antibiotic treatment.