SAN FRANCISCO, CA—Weight-based vancomycin dosing may provide a higher rate of adequate concentration for an entire surgical procedure—from the time of incision until complete wound closure—in patients undergoing high-risk orthopedic surgery, results of a study presented at IDWeek 2013 have found.

The objective of the study was “to demonstrate potential benefits of vancomycin weight-based dosing compared to traditional vancomycin 1g” dosing in this patient population, Marco R. Scipione, PharmD, of NYU Langone Medical Center, New York, NY, and colleagues reported.

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Since 2008, all patients undergoing arthroplasty and spine fusion at the medical center are screened for Staphylococcus aureus nasal colonization. Vancomycin is added to cefazolin if methicillin-resistant S. aureus (MRSA) is detected.

A total of 216 patients with positive MRSA nasal screen and arthroplasty or spine fusion performed between January 2009–2012 were included in the analysis. All patients received vancomycin 1g within 1 hour prior to incision. A vancomycin weight-based dose (15mg/kg of actual body weight [ABW], rounded to nearest 250mg to a maximum of 2g) was also calculated for each patient.

Patients were categorized as underdosed if the calculated weight-based dose was >1g dose or overdosed if the calculated weight-based dose was <1g dose. “Pharmacokinetic formulas were used to estimate vancomycin levels at the time of incision (peak level) and time of wound closure based on duration of the procedure,” they stated.

Mean age of the patients was 60 years and 48% were male. The majority of the patients (68%) underwent arthroplasty; 24%, spine fusion; and 8%, laminectomy. Mean ABW was 86kg with 68% of patients having ABW >20% of ideal body weight.

A vancomycin 1g dose was appropriate in 21% of patients; 10% were overdosed by 250mg and 69% were underdosed (30% by 250mg, 22% by 500mg, 12% by 750mg, and 5% by 1000mg). 

Estimated levels at the time of incision would be >20mg/L in 25.5% of patients for the 1g dose vs. 82.4% for patients dosed on ABW, and at the end of procedure, <15mg/L in 60.2% of patients with 1g compared to 11.6% with weight-based dosing (P=0.0002).

Study analysis showed that 9 patients developed postoperative surgical site infections (PSSI), 6 of whom had positive MRSA cultures. These 6 patients were underdosed with vancomycin 1g by 250mg (n=2) and 500mg (n=4); 5 of the 6 patients had an estimated level at wound closure with 1g of <15mg/L.

“Our analysis demonstrated that due to patient variability in weight and the increasing vancomycin MIC of MRSA, the traditional vancomycin 1g dose could result in underdosing and inadequate dosing in the majority of patients,” Dr. Scipione noted. “Our surgical prophylaxis guidelines have been changed to recommend vancomycin weight-based dosing.”