Classification of peritonitis by stage.
| Stage | Description |
| I | Diffuse purulent peritonitis; no organ involvement |
| II | Purulent peritonitis with involvement of an organ (renal failure, respiratory insufficiency/disruption of external (mechanical) respiration leading to abnormally altered blood gases, etc.). |
| III | Purulent peritonitis with involvement of ≥ 2 organs |
