Peritonitis is a severe complication after abdominal surgery. Patients admitted at the Intensive Care Unit (ICU) following surgery bear the risk of localized infection, sepsis or septic shock. Prevention or early detection of such events is important to intervene with an appropriate therapeutic action and avoid risking a potentially life-threatening situation. White blood cell counts (WCC) and C-Reactive Protein (CRP), Interleucin-6 (IL-6) and Procalcitonin (PCT) have all been promising parameters, however, they are useful only in selective cases and have a limited diagnostic accuracy.