Peritonitis refers to inflammation (usually infectious) of the peritoneum. Infectious peritonitis is broken into three classes, although the terms don’t necessarily make sense:
- Primary peritonitis: also called spontaneous bacterial peritonitis (SBP), this is a monomicrobial infection of ascites usually caused by enteric bacteria and Strep. pneumoniae. It can present with minimal or no abdominal symptoms, so it should be considered in almost everyone who has ascites and is unwell. Ascitic fluid classically has more than 250 neutrophils and a positive culture, but either of these alone also constitutes primary peritonitis. It’s called primary because the source of the infection is unclear*.
- Secondary peritonitis: this is an intraabdominal infection secondary to a clear process, usually trauma or surgical misadventure. The usual bacteria is whatever is in poop (so your enteric Gram-negatives and some Gram-positives).
- Tertiary peritonitis: this is what we call secondary peritonitis when it’s gone on for too long. The organisms that grow in culture are often low-virulence bacteria that may or may not be contaminant or colonization, though yeast and resistant organisms can also be found. Basically, it’s secondary peritonitis after you’ve killed off all of the usual suspects with course of broad-spectrum antibiotics.
* It’s poop. It’s just not by direct spread, like you might think, but rather translocation of bacteria into the lymphatics, which drains into the bloodstream via the thoracic duct, then hematogenously seeds the ascitic fluid.
Read more: Chapter 76: Peritonitis and Intraperitoneal Abscesses, in Mandell Principles and Practice of Infectious Diseases 8e.