C-reactive protein in the first 30 postoperative days and its discriminative value as a marker for postoperative infections, a multicentre cohort study

OBJECTIVE: To assess the association of C-reactive protein (CRP) with postoperative infections for eight different types of surgery using big data. DESIGN: A multicentre cohort study with longitudinally collected data from electronic health records, collected from 1 January 2011 to 22 September 2023. SETTING: Data of two tertiary medical centres in the Netherlands were used. PARTICIPANTS: This study included all procedures (42 125 in total) in adult patients undergoing surgery in two tertiary medical centres in the Netherlands. OUTCOME MEASURES: The primary outcome was the association between CRP and a postoperative infection in the first 30 days postoperatively. Postoperative infection was defined by an action-based definition, that is, patients had to be treated for an infection with anti-microbial treatment and/or an intervention (eg, surgical drainage) to be classified as having a postoperative infection. CRP measurements were divided into a reference group (0-5.0 mg/dL) and four groups for comparison (5.1-10.0 mg/dL, 10.1-15.0 mg/dL, 15.1-20.0 mg/dL and >20.0 mg/dL). Subgroup analyses were performed for eight major surgical subspecialties and for the two medical centres separately. RESULTS: A total of 175,779 CRP measurements were performed, of which the majority was drawn in the first postoperative week. The ORs for developing a postoperative infection varied between 1.0 (0.9-1.1 95% CI) and 12.0 (9.5-15.1 95% CI), with a stronger association for the higher level of CRP categories and when more time had elapsed since surgery. Sensitivity ranged between 11% and 34%, specificity ranged between 64 and 95%, and the positive and negative predicting value ranged between 12% and 51% and 88% and 94%, respectively. For the surgical subspecialties and the two hospitals separately, similar results were found. CONCLUSION: In this study, an elevated postoperative CRP was associated with postoperative infections with a stronger association for higher CRP levels. The association was stronger if a longer time had elapsed since surgery, which contrasts with the moment most CRP measurements were done, namely in the first postoperative week. Clinicians should take the evolving value of CRP in mind when using it in the diagnosis of postoperative infections.