Risk factors for treatment failure of surgical site deep infections following spinal instrumentation: multicentre observational study (DeFEndO study)

Introduction: To describe the characteristics and outcome of patients diagnosed with deep surgical-site infections following spinal instrumentation (DSIIs) and to investigate risk factors for treatment failure including antimicrobial treatment duration as the main exposure. Materials and methods: An observational retrospective study on patients diagnosed with DSII from January 2018 to June 2022 in two large hospitals. Two multivariable analyses were carried out to evaluate risk factors for treatment failure considering two treatment schedules: 8-week versus 12-week. Results: We enrolled 161 patients, median age 62 years (IQR: 51-69), 55.1 % male, median Charlson comorbidity index of 3 (IQR: 1-4). Gram-positive and Gram-negative bacteria were responsible for DSII in 60.3 % and 33.8 % of cases. Polymicrobial infections accounted for 42 cases (26.4 %). Treatment failure was observed in 51 (31.7 %) patients. Multivariable analyses were conducted to evaluate risk factors for treatment failure using treatment duration cutoffs of 8 weeks and 12 weeks. Quinolone administration was associated with a lower risk of treatment failure in both treatment schedules (8 weeks: OR 0.11, 95 % CI 0.02-0.71, p=0.020; 12 weeks: OR 0.11, 95 % CI 0.02-0.64, p=0.015). Gram-negative aetiology of DSII was a risk factor for treatment failure (8 weeks: OR 4.20, 95 % CI 1.08-16.38, p=0.039; 12 weeks: OR 4.46, 95 % CI 1.15-17.23, p=0.030). Conclusions: DSII caused by Gram-negative bacteria was associated with a higher risk of worse outcomes. Quinolone use appeared to be associated with a lower risk of treatment failure, but this finding should be interpreted with caution given the potential for residual confounding. No significant association between treatment duration and outcome was observed in our exploratory analyses.