Preoperative C-reactive protein stratifies risk of intra-abdominal infection within AAST grade I appendicitis: a multicenter cohort study of 5899 patients

The American Association for the Surgery of Trauma (AAST) operative grading system classifies the severity of appendicitis at the time of surgery. Even within AAST operative grade I appendicitis, postoperative intra-abdominal infectious complications (IAI) can occur, but the risk heterogeneity within this category remains poorly characterized. We conducted a multicenter retrospective study of adults who underwent appendectomy for AAST operative grade I appendicitis at three university-affiliated hospitals. Free-text radiology, operative, and pathology reports were processed using negation-aware keyword extraction. The primary outcome was IAI within 30 days, defined as a composite of intra-abdominal abscess, stump leak, and peritonitis. Among 5,899 patients, IAI occurred in 208 (3.5%). Preoperative C-reactive protein (CRP), albumin, and platelet count were independently associated with IAI, but only CRP provided a significant discriminatory contribution (AUC 0.582 to 0.680, P < 0.001). The Youden-optimal cutpoint for CRP was 49.7 mg/L (with 94.2% of 1,000 bootstrap replicates falling within 40–60 mg/L). The high-CRP group (≥ 50 mg/L; n = 1,336) had a markedly higher IAI rate (7.9% vs. 2.3%; adjusted OR 3.40, 95% CI 2.54–4.56). This group was also enriched for radiologic features of periappendiceal inflammatory extension and intraoperative findings of advanced local inflammation. The association remained consistent across operative–pathologic concordance, hospital type, and treatment era ( P > 0.20 for all interactions). In patients with AAST grade I appendicitis, an elevated preoperative CRP was associated with a higher risk of IAI and with features of more advanced local inflammation or subtle complicated features not apparent at operation. This suggests that CRP may help refine risk stratification within a category currently treated as homogeneous.