Serum and local vancomycin concentrations following intrawound application in elective lumbar spinal fusion: A secondary pharmacokinetic analysis of the vancomycin arm of a prospective randomized controlled trial

Background: Intrawound vancomycin powder is widely used as an adjunct to systemic prophylaxis in spine surgery, yet prospective pharmacokinetic data are limited and concerns about systemic absorption persist. We report the prespecified pharmacokinetic endpoints of a randomized controlled trial, quantifying serum and local wound-drain vancomycin concentrations after intrawound application. Methods: This is a secondary pharmacokinetic analysis of the vancomycin arm of a prospective, patient-blinded, randomized controlled trial. The 140 patients in this arm underwent elective 1- or 2-level posterior lumbar interbody fusion (August 2015-April 2019) and received 1 g intrawound vancomycin powder additional to routine i.v. cefuroxime prophylaxis. Serum vancomycin was measured on postoperative days 1, 2, 3, and 5 and wound-drain concentrations at 24 and 48 hours. Results: Serum vancomycin was available for 106 to 110 of 140 patients on days 1 to 2 and remained negligible (day 1: 0.10 ± 0.48 µg/mL; day 5: 0.00 µg/mL); it was detectable in 7 of 106 patients (6.6%) on day 1, and no value exceeded 4.20 µg/mL. Wound-drain concentrations (104-105 of 140 patients) were supratherapeutic (day 1: 969.33 ± 798.08 µg/mL; day 2: 709.01 ± 470.23 µg/mL), with 100% of samples above the MRSA MIC₉₀ (2 µg/mL). Conclusions: In elective lumbar fusion, 1 g intrawound vancomycin produced supratherapeutic local wound-fluid concentrations with negligible systemic absorption, supporting its rationale as an adjunct to systemic prophylaxis.