The diagnosis of intra-abdominal infection and post-operative peritonitis based on clinical examination, biomarkers and radiological signs, should be made as early as possible to improve outcomes and decrease mortality through early and optimal source control, adequate surgery and appropriate antibiotic therapy (Montravers et al. Therapeutic management of peritonitis: a comprehensive guide for intensivists. Intensive Care Med 2016;42:1234–47). However, the indication and the timing of the surgery is often not an easy decision. This case presents the use of a novel early biomarker of infection and sepsis, pancreatic stone protein (Fidalgo et al. Pancreatic stone protein: review of a new biomarker in sepsis. J Clin Med 2022;11:1085), as a tool to aid in the diagnosis of intra-abdominal infection and post-operative peritonitis and to help guide the decision for adequate surgeries in a patient with intra-abdominal infection and post radical prostatectomy peritonitis.