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A methodological systematic review.To critically appraise the validity of preventive effects attributed to prophylactic treatments for surgical site infection (SSI) after spinal surgery.As a result of a rapidly increasing number of spinal procedures, health care expenditure is expected to increase substantially in the foreseeable future. Administration of effective prophylactic treatments may prevent occurrence of SSIs and may thus result in lower costs. To date, however, no review appraising the methodological quality of studies evaluating prophylactic treatments for spinal SSIs has been published.Contemporary studies evaluating the preventive effect of prophylactic interventions on the rate of SSI after spinal surgery were searched through the Medline and EMBASE databases (January 2001 to December 2010). References were retrieved and bias-prone study features were abstracted individually and independently by 2 authors.Eighteen eligible studies were identified, including 6 randomized controlled trials and 12 comparative cohort studies. Most often, antibiotic prophylaxis administration was investigated (n = 7). Included studies covered a wide variation of indications and surgical procedures. Except for 5 studies (28%), applied definitions of SSI outcomes were ambiguous. Although several important methodological aspects, including blinding of outcome assessors and attrition, were poorly reported in randomized controlled trials, these studies were far less susceptible to bias and confounding as observed in nonrandomized studies. None of the 12 cohort studies adjusted for confounding by matching, stratification, or multivariate regression techniques.Given the plethora of previously hypothesized confounding risk factors for a spinal SSI, conduct of nonrandomized comparative therapeutic studies is strongly discouraged. On the other hand, methodological safeguards, including use of standardized definitions of putative confounders and outcomes, should be considered in more detail during the design phase of a randomized trial.