Once peritonitis occurs, it can be extremely difficult to treat. For this reason, prompt diagnosis is crucial to an overall positive prognosis.

A physician will perform a complete physical examination and compile a thorough medical history. The physician will pay special attention to the abdomen, applying gentle pressure and quickly releasing it to see if the pain worsens, indicating an inflamed peritoneum.

If the physician suspects peritonitis, x-rays may be taken of the patient lying down and standing up. These images can reveal any free gas in the abdominal cavity resulting from a rupture. In some cases, a needle may be used to withdraw fluid from the abdominal cavity. This will allow testing to identify the nature of the infectious organism (whether it is naturally occurring within the body or foreign) and to test how it reacts to different types of antibiotics. Contrast-enhanced  CAT scansCAT scansCAT scans can often help identify the intraabdominal source of a primary bacterial peritonitis  infection.

Other diagnostic tests may include a culture, chemical examination, analysis (paracentesis) or cytologic examination (cell study) of the peritoneal fluid. Blood testing may also be performed to check for an elevated white blood cell count, which may indicate infection.

In addition, researchers have found that monitoring the levels of a chemical hormone in the bloodstream may help to more easily identify peritonitis and other serious abdominal infections that occur after surgery.