Peritonitis refers to inflammation of the peritoneum, which is the lining of the abdomen.

Peritonitis is a really important clinical sign that refers to inflammation of the lining of the abdomen. The peritoneum is a serous membrane that essentially lines the abdominal cavity. It is composed of two layers and a potential space:

  • Parietal peritoneum: lines the abdominal cavity. Boundaries are the pelvis inferiorly, retroperitoneal space posteriorly, diaphragm superiorly, and abdominal wall anteriorly
  • Visceral peritoneum: wraps around the visceral (internal) organs located within the intraperitoneal space
  • Peritoneal cavity: a potential space located between the parietal and visceral peritoneum. It contains a small amount of fluid (~50 mL)

Peritonitis is most commonly caused by perforation of an abdominal viscera (e.g. appendix, colon, gallbladder). It classically causes ‘peritonism’, which refers to the signs and symptoms of peritonitis that principally includes severe abdominal pain that is worse on movement and associated with percussion tenderness and rigidity (discussed below). Other common features are nausea, vomiting, fever, and tachycardia.

Peritonitis is one of the major differentials in a patient presenting with acute abdominal pain (often known as ‘the acute abdomen’) and a common reason for referral to the general surgical team. Timely recognition, imaging, and treatment are vital.


There are several important terms that are used to describe and classify peritonitis.

Peritonitis can be classified into primary, secondary, or tertiary:

  • Primary: spontaneous bacterial invasion of the peritoneal cavity. Also known as spontaneous bacterial peritonitis. Seen in patients with pre-existing ascites (mainly chronic liver disease)
  • Secondary: peritoneal infection due to loss of integrity of the gastrointestinal or urogenital tracts. This leads to contamination of the peritoneal space. This is the cause in perforation
  • Tertiary: recurrent or persistent infection of the peritoneal cavity that typically occurs after secondary peritonitis. Less well defined and usually seen in patients who are immunocompromised

Inflammation of the peritoneum may be localised or generalised:

  • Localised: this refers to a focus of infection that is usually walled-off or contained by adjacent organs
  • Generalised: also known as diffuse, this refers to an infection that has spread throughout the entire cavity

Peritonitis is most commonly caused by infections, but rarely, aseptic peritonitis can develop in the absence of infection. This may be seen in some autoimmune diseases such as systemic lupus erythematosus.

Aetiology & pathophysiology

Peritonitis is most commonly caused by gastrointestinal perforation.

A perforation affecting any of the major gastrointestinal organs can lead to secondary peritonitis. Content spills out into the normally sterile field leading to a localised or generalised infection. Urogenital pathologies can also lead to peritonitis.

Commonly identified causes include a perforated appendix, diverticulum in the colon, or peptic ulcer. Sites of perforation are listed below:

  • Oesophagus: penetrating injury (trauma), rupture (Boerhaave syndrome), malignancy
  • Stomach: malignancy, peptic ulcers
  • Pancreas: pancreatitis
  • Hepatobiliary: gallstones, cholecystitis, malignancy
  • Small intestines: ischaemic bowel, strangulated hernias
  • Large intestines: diverticulitis, colorectal cancer

The spillage of content from the gastrointestinal (or urogenital) tracts allows the entry of bacteria into the previously sterile compartment. Commonly encountered bacteria include gram-negative (e.g. Escherichia coli and Klebsiella pneumoniae) and anaerobic bacteria. The actual pathogens differ depending on the organ affected with gram-positive organisms observed with upper GI perforations.

A localised infection may develop because the infection becomes walled-off or is contained by the positioning of adjacent organs. In this situation, an abscess may develop in an attempt to control the spread of infection. If the infection spreads throughout the peritoneal cavity this leads to generalised peritonitis. Older patients, those who are immunosuppressed, or those with additional risk factors, are at higher risk of generalised peritonitis with increased risk of major complications including septic shock and multi-organ dysfunction syndrome.

For more information on primary peritonitis (i.e. spontaneous bacterial peritonitis) see our Ascitic fluid note.

Clinical features

The cardinal feature of peritonitis is acute, severe abdominal pain.

Peritonitis should be suspected in any patient with sudden onset, severe abdominal pain. Accompanying features are usually fever, nausea, vomiting, and systemic upset (e.g. tachycardia, hypotension). Patients may be extremely unwell with shock.

Abdominal pain is often dull and poorly localised initially due to inflammation of the visceral peritoneum. As the parietal peritoneum becomes involved, pain becomes more severe, sharp, and localised (e.g. acute appendicitis causing periumbilical pain that subsequently migrates to the right iliac fossa).


  • Abdominal pain: may be localised or generalised
  • Pain worse on movement: patients often described as wanting to stay rigid to prevent aggravating the inflammation
  • Abdominal distension
  • Anorexia
  • Nausea & vomiting
  • Fever
  • Sweating


  • Tenderness on palpation
  • Guarding: voluntary contraction of the abdominal muscles (to protect from the pain)
  • Rigidity: involuntary muscle contraction due to underlying inflammation
  • Percussion tenderness: pain on light percussion over the affected area (a modern version of rebound tenderness)
  • Fever
  • Tachycardia
  • Absent/reduced bowel sounds
  • Inflammatory mass: may suggest an abscess
  • Features of septic shock: hypotension, oliguria/anuria, confusion, mottled skin

NOTE: make sure you check the hernial orifices in a patient with suspected peritonitis to exclude a strangulated hernia.

Diagnosis & investigations

Peritonitis is usually a clinical diagnosis that is confirmed with imaging in modern healthcare.

Peritonitis is classically a clinical diagnosis based on a typical history of acute, severe abdominal pain with peritonism on examination (i.e. guarding, rigidity, rebound tenderness).

Nonetheless, the role of imaging (particularly CT) has been increasing in patients presenting with an ‘acute abdomen’ due to the ease of access to CT within the UK. Consequently, many patients will undergo a CT to evaluate the cause of acute abdominal pain, including suspected peritonitis, before considering surgical intervention. In some cases, it may be more appropriate to take patients straight to theatre as the clinical history and examination are clear cut to warrant urgent surgical intervention.

Bloods tests

Patients presenting with suspected peritonitis need urgent blood tests. These help towards the suspected diagnosis (e.g. raised inflammatory markers), determine the extent of organ dysfunction/hypoperfusion (e.g. lactate), and ensure it is safe to proceed to an operation if necessary.

Typical tests include:

  • Full blood count
  • Renal profile
  • Liver function tests
  • Venous/arterial blood gas
  • CRP
  • Coagulation
  • Group & save +/- Crossmatch


Imaging has become a mainstay investigation of the ‘acute abdomen’ and suspected peritonitis. In modern healthcare, CT is often used as the first-line investigation to assess the whole of the intra-abdominal and pelvic organs. This enables the location of the site of the pathology and provides objective evidence of a perforated viscus by finding free air and/or fluid with the abdominal cavity.

  • X-rays: plain film radiographs may be requested in patients presenting with peritonitis, but use has largely been superseded by CT. Abdominal and erect (e.g. standing upright) x-rays are often requested to look for free air (particularly under the diaphragm) as a marker of perforation. However, small amounts of air may be easily missed
  • CT: this is the predominant investigation for the acute abdomen. It is quick and enables imaging of the whole abdomen and pelvis. It can be used to evaluate for many different things simultaneously including free air/fluid, areas of inflammation, ischaemia, and abscess formation. Due to radiation and the risk of contrast, it may be avoided in young patients, pregnant patients, and those with severe renal impairment. However, a risk/benefit decision may be required in some cases
  • US: ultrasound may be used in the acute setting to identify free fluid in the abdomen, particularly in the setting of trauma. Bedside assessment with US enables rapid decision-making. However, small amounts of fluid may not be detected on ultrasound and its use is operator dependent


Patients with peritonitis often require urgent surgical exploration.

Any patient with suspected peritonitis should be urgently referred to the appropriate surgical team for further management, which commonly involves surgical intervention. Intravenous antibiotics and supportive measures (i.e. monitoring, intravenous fluids, inotropes) are essential to management.


Broad-spectrum, high-dose, antibiotics are required in patients with suspected peritonitis and local antibiotics guidelines should be followed for treating intra-abdominal sepsis. This commonly involves co-amoxiclav, gentamicin, cefuroxime, and/or metronidazole. These antibiotics are often used in combination to provide adequate cover against a range of potential bacteria (particularly gram-negative and anaerobic).

Supportive treatment

Patients with peritonitis may have features of sepsis with major organ dysfunction. It is critical to provide supportive treatment for organ dysfunction that may involve respiratory support (e.g. oxygen, non-invasive ventilation), cardiovascular support (e.g. fluids, inotropes), renal support (e.g. fluids, dialysis if needed), etc. Patients who are critically unwell with septic shock of multi-organ dysfunction syndrome will usually require ongoing management in an intensive care setting.

Surgical intervention

The management of peritonitis depends on whether it is primary or secondary, localised or generalised, and on the suspected underlying cause. In those who need urgent surgical intervention, an exploratory laparotomy is commonly performed. This involved a large midline incision to enable access to the abdominal cavity. The role of surgery is to remove the source of contamination, provide local control through wash-outs and repair any anatomical or functional defect. In certain situations, repeated surgical interventions may be required to provide adequate source control.

In patients who are critically unwell, management may centre on damage limitation surgery to quickly control the source of infection and defer any definite repair until a second stage operation when the patient is more stable.

Radiological intervention

Patients with localised peritonitis and the development of abscesses may be amenable to radiological drainage rather than surgical intervention. This depends on the size and site of any collection and should be discussed with the interventional radiology team.

Last updated: July 2022
Author The Pulsenotes Team A dedicated team of UK doctors who want to make learning medicine beautifully simple.

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