Phlegmonous Appendicitis With Normal Bilirubin and Procalcitonin: Evidence of Compartmentalized Inflammation

Acute appendicitis is commonly evaluated using systemic inflammatory biomarkers, including C-reactive protein, procalcitonin, and serum bilirubin, to estimate disease severity and guide clinical decision-making. However, the relationship between local pathological severity and systemic biomarker response is not always consistent. We report a case of a 48-year-old premenopausal woman presenting with clinical features of acute appendicitis and elevated inflammatory markers, including C-reactive protein, but with normal serum bilirubin and procalcitonin levels. The patient underwent appendectomy, and intraoperative findings were consistent with acute inflammation without perforation. Histopathological examination confirmed acute phlegmonous appendicitis with transmural neutrophilic infiltration and localized fibrinopurulent peritonitis. Microbiological cultures yielded Bacteroides fragilis and Enterococcus species, supporting a polymicrobial localized infection. Incidental appendiceal endometriosis was identified in the subserosal region and confirmed by immunohistochemistry (CK7, ER, PR, CD10). Despite advanced local inflammation, the absence of hyperbilirubinemia and normal procalcitonin levels suggests limited systemic inflammatory involvement in this case. This observation supports the concept of compartmentalized inflammation in acute appendicitis, where severe local disease may occur without systemic biomarker elevation. Clinically, this highlights that normal bilirubin and procalcitonin values do not exclude advanced appendiceal inflammation and should be interpreted cautiously. The coexistence of appendiceal endometriosis appears incidental, reflecting the complexity of local pathological substrates rather than a definitive etiological factor. This case underscores the importance of integrating clinical, laboratory, and histopathological findings in the assessment of appendicitis severity.