Do Prophylactic Antibiotics Reach the Operative Site Adequately?: A Quantitative Analysis of Serum and Wound Concentrations of Systemic and Local Prophylactic Antibiotics in Spine Surgery

STUDY DESIGN: Prospective cohort study. OBJECTIVE: To analyze the serum and drain concentrations of antibiotics administered by two different routes and compare the results. SUMMARY OF BACKGROUND DATA: Systemic antibiotics are expected to reach the surgical site and maintain adequate concentrations of the drug to prevent infection. However, it is unknown whether systemically administered antibiotics reach and maintain such adequate concentrations at the surgical wound or not. METHODS: Forty patients undergoing elective spine surgery received intra-wound Vancomycin (1 GM) before the wound closure and single dose of intravenous Gentamycin (80MG) immediately after surgery. Blood and drain samples were collected postoperatively to estimate serum and drain concentrations of Gentamycin and Vancomycin. Drug Estimation Protocol: Drug concentrations were estimated by ADVIA Centaur CP immunoassay (direct chemiluminescence). Gentamycin and vancomycin in the test samples competes with their respective acridinium ester-labeled gentamicin and vancomycin derivatives for monoclonal mouse anti-gentamycin and anti-vancomycin antibodies which are covalently coupled to paramagnetic particles in the solid phase. RESULTS: Gentamycin attained peak serum levels at 6 hours following administration with an average value of 9.90 +/- 3.1 mug/mL which was decreased to 6.76 +/- 2.6 mug/mL at 12 hours and steadily declining thereafter. Even though, the drug concentrations in the drain collection from the wound also attained peak levels at 6 hours, the drug concentrations were lower (3.75 +/- 1.4 mug/mL) than that of serum concentrations and inadequately attained the recommended target peak of Gentamycin (4-12 mug/mL).Wound levels of local vancomycin were significantly higher at 6 hours (413.4 +/- 217.3 mug/mL) and well maintained even at 72 hours. Serum vancomycin levels were observed to be highest at 6 hours in negligible concentrations of 6.06 +/- 2.2 mug/mL. CONCLUSION: After prophylactic systemic administration of the antibiotics, the antibiotic drug concentrations in the wound are much lower than the serum concentrations at any given time. After local intra-wound application of antibiotics, the drug concentrations in the wound are well maintained even after 72 hours. LEVEL OF EVIDENCE: 3.