In simple words: Peritonitis is an inflammation of the peritoneum, the thin tissue that lines the inner wall of the abdomen and covers most of the abdominal organs.

Peritonitis. Excludes1: acute appendicitis with generalized peritonitis (K35.2-), aseptic peritonitis (T81.6), benign paroxysmal peritonitis (E85.0), chemical peritonitis (T81.6), gonococcal peritonitis (A54.85), neonatal peritonitis (P78.0-P78.1), pelvic peritonitis, female (N73.3-N73.5), periodic familial peritonitis (E85.0), peritonitis due to talc or other foreign substance (T81.6), peritonitis in chlamydia (A74.81), peritonitis in diphtheria (A36.89), peritonitis in syphilis (late) (A52.74), peritonitis in tuberculosis (A18.31), peritonitis with or following abortion or ectopic or molar pregnancy (O00-O07, O08.0), peritonitis with or following appendicitis (K35.-), puerperal peritonitis (O85), retroperitoneal infections (K68.-)

Example 1: A patient presents with abdominal pain, fever, and tenderness. After examination and diagnostic testing, they are diagnosed with spontaneous bacterial peritonitis (SBP), which is coded as K65.0., A patient with a ruptured appendix develops generalized peritonitis. This would be coded as K35.2, as the primary diagnosis is appendicitis with peritonitis as a complication. K65 would not be used in this case., A patient undergoing peritoneal dialysis develops peritonitis related to the procedure. This would be classified under T81.6 (Peritoneal adhesions, not elsewhere classified) or other relevant complication codes for dialysis, rather than K65.

Documentation should include signs and symptoms, laboratory results (such as elevated white blood cell count, positive cultures), imaging findings (if applicable), and the underlying cause of peritonitis.If an infectious agent is identified, it should be documented and coded with the appropriate B95-B97 code.