Systemic antibiotic prophylaxis prior to gastrointestinal surgery - is oral administration of doxycycline and metronidazole adequate?

Background: Antibiotic prophylaxis is recommended prior to a wide range of gastrointestinal operations to reduce the rate of surgical site infections (SSIs). Traditional intravenous (IV) drugs are costly and their preparation strains nursing resources at the wards. While oral administration may attenuate these limitations, its use remains limited. We aimed to assess whether a dual oral antibiotic prophylaxis regimen provides adequate serum concentrations throughout the surgical procedure. Methods: We measured serum concentrations of doxycycline and metronidazole following single oral doses of 400mg doxycycline and 1200mg metronidazole at first incision and repeated at wound closure in a cohort of patients undergoing elective gastrointestinal surgery. Both drugs were dispensed at least two hours before skin incision. Serum concentrations were compared to minimum inhibitory concentrations (MIC) and epidemiological cut-off values (ECOFFs) for relevant pathogens. Results: Mean serum concentrations of doxycycline at first incision and at wound closure were 5.75mg/L and 4.66mg/L and of metronidazole 18.88mg/L and 15.56mg/L, respectively. Metronidazole concentrations were above ECOFF (2mg/L) for relevant anaerobic species in 103/104 of patients in both samples. Doxycycline serum concentrations were above the ECOFF for common Enterobacteriaceae species (4mg/L) in both samples in 58/104 patients (55.8%). Conclusions: A single dose of orally administered metronidazole provides adequate concentrations throughout surgery in a heterogeneous cohort of patients. Uncertainty p ersists regarding the adequacy of doxycycline concentrations, as the optimal serum level of doxycycline in a prophylactic setting has not been established.