Peritonitis is inflammation of the localized or generalized peritoneum, the lining of the inner wall of the abdomen and cover of the abdominal organs.

[1] The main manifestations of peritonitis are acute abdominal pain, abdominal tenderness, abdominal guarding, rigidity, which are exacerbated by moving the peritoneum, e.g., coughing (forced cough may be used as a test), flexing one's hips, or eliciting the Blumberg's sign (meaning that pressing a hand on the abdomen elicits less pain than releasing the hand abruptly, which will aggravate the pain, as the peritoneum snaps back into place).

In either case, pain typically starts as a generalized abdominal pain (with involvement of poorly localizing visceral innervation of the visceral peritoneal layer), and may become localized later (with involvement of the somatic innervation of the parietal peritoneal layer).

Rigidity (involuntary contraction of the abdominal muscles) is the most specific exam finding for diagnosing peritonitis.

If diffuse peritonitis is detected, then urgent surgical consultation should be obtained, and may warrant surgery without further investigations.

In people with ascites, a diagnosis of peritonitis is made via paracentesis (abdominal tap): More than 250 polymorphonuclear cells per μL is considered diagnostic.

[15][16] In normal conditions, the peritoneum appears greyish and glistening; it becomes dull 2–4 hours after the onset of peritonitis, initially with scarce serous or slightly turbid fluid.