The effect of under-dosing prophylactic antibiotics in the care of open tibial fractures

Objectives: To determine the frequency and effect of under-dosing prophylactic weight-based antibiotics in patients with open tibial fractures. We hypothesized that patients who did not receive appropriate weight-based dosing of prophylactic antibiotics would have higher rates of infection. Design: Retrospective cohort study. Setting: Level 1 Trauma Center. Patients/Participants: Patients 18 years of age or older with high-grade (Gustilo-Anderson type IIIA or IIIB) open extraarticular tibial fractures over a 5-year period. Main Outcome: The primary outcome was deep infection within one year of initial injury. Appropriate weight-based dosing of cefazolin was defined as: At least 1 g for patients \u3c80 kg, 2 g for patients between 80 and 120 kg, and 3 g for patients \u3e120 kg. Results: Sixty-three patients met the inclusion criteria; 21 (33%) were under-dosed with cefazolin at the time of initial presentation. Among the 20 patients who subsequently developed deep infection, only 55% were appropriately dosed with cefazolin; of the patients who did not develop deep infection, 72% were appropriately dosed with cefazolin (P = 0.18). Univariate analysis revealed that hypertension was associated with infection (P = 0.049). Multivariable logistic regression analysis of infection due to all organisms did not reveal a statistically significant reduction in the odds of infection with appropriate weight-based dosing of cefazolin [Odds ratio = 0.42 (95% confidence interval, 0.12-1.48), P = 0.177]. Five of 7 (71%) of the gram positive, non-methicillin-resistant Staphylococcus aureus, infections occurred in patients who were under-dosed with cefazolin. Five (23.8%) of 21 patients who were under-dosed with cefazolin had gram-positive, non-methicillin-resistant S. aureus infections, compared to 2 (4.8%) of 42 patients who were appropriately dosed (P = 0.036). Conclusions: Under-dosing of weight-based antibiotics in the treatment of open fractures is common. Appropriate weight-based dosing of cefazolin for prophylaxis in high-grade open tibial fractures reduces the frequency of infection due to cefazolin-sensitive organisms. Interestingly, organisms not susceptible to cefazolin were responsible for the majority of infections. The effect of under-dosing of cefazolin and other weight-based antibiotics deserves further investigation in larger studies