Is it valid to compare surgical site infections rates between countries? Insights from a study of English and Norwegian Surveillance Systems

OBJECTIVE To assess whether differences in surveillance methods or underlying populations significantly influence internationally reported national SSI rates by comparing surveillance data from 2 countries. DESIGN Retrospective cohort. SETTING England and Norway. METHODS We assessed the population under surveillance and surveillance methodology to compare SSI rates in 2 countries (September 2012-January 2015) for 4 surgical categories: Coronary artery bypass graft (CABG), colon surgery, cholecystectomy, and hip prosthesis (HPRO). We compared the inpatient SSI incidence using logistic regression, adjusting for the following known risk factors: Sex, age, ASA score, wound class, postoperative hospital days, and operation duration. Subsequently, we restricted further analyses to the procedures reported by both countries. RESULTS There were important differences in case definitions for superficial infection, so we restricted our analyses to deep incisional and organ-space SSIs. For CABG, the crude odds ratio (OR) for England compared to Norway was 2.4 (95% CI, 1.4-4.4), whereas adjusted OR (aOR) lost significance (aOR, 1.1; 95% CI, 0.57-2.0). For colon surgery the decreased odds (OR, 0.68; 95% CI, 0.56-0.81) remained significant after adjustment (aOR, 0.42; 95% CI, 0.34-0.51). We found no associations for cholecystectomy. For HPRO, the crude OR suggested no significant difference (OR, 1.2; 95% CI, 0.72-2.1), whereas the aOR was significantly lower in England (aOR, 0.45; 95% CI, 0.25-0.81). Including only the subset of procedures reported by both countries yielded comparable results. CONCLUSION Differences in case definitions and population under surveillance in the English and Norwegian SSI surveillance systems affected SSI estimates, making the comparison of crude rates unreliable. Standardized definitions and adjustment for established risk factors are essential for European comparisons to guide related public health actions.