General Surgery

Peritonitis

Bacteria or gut contents reach the peritoneal cavity, which cannot clear them, so infection spreads across its huge surface and drives ileus, third-space fluid loss and sepsis.

In a nutshell

Peritonitis is infection of the peritoneal lining in one of three settings: a perforated or infected abdominal organ, cirrhotic ascites, or peritoneal dialysis. Resuscitate on the sepsis pathway and give broad-spectrum intravenous antibiotics within 1 hour. Then work out which of the three it is: the drug, the route and the definitive treatment all differ.

Classic presentation

A patient lying completely still with constant abdominal pain, board-like rigidity, absent bowel sounds and a rising heart rate, two days into an episode of left iliac fossa pain.

Key points

Antibiotics never fix an uncontrolled source. If there is an ongoing leak, ischaemic bowel or an undrained collection, the definitive treatment is surgery or drainage.
Spontaneous bacterial peritonitis (SBP) is diagnosed on an ascitic neutrophil count above 250 cells/mm³, and is treated even when the culture grows nothing.
Albumin 1.5 g/kg within 6 hours and 1 g/kg on day 3 is given in SBP when serum creatinine is raised or rising.
In peritoneal dialysis (PD) peritonitis the antibiotics go into the dialysate, not a vein, unless the patient is systemically septic.
Norfloxacin is the trial-proven drug for preventing recurrent SBP but is not available in the UK, so ciprofloxacin 500 mg once daily is used instead.
A NEWS2 (National Early Warning Score 2) score of 7 or more is high risk and starts a 1-hour clock for broad-spectrum intravenous antibiotics.
Diverticular abscesses larger than 3 cm are drained percutaneously or surgically; smaller ones are treated with antibiotics and stepped down to oral.
PD effluent that has not cleared after 5 days of appropriate antibiotics means the catheter comes out.

First-line investigation

Blood gas with lactate plus the sepsis blood set, then contrast-enhanced CT of the abdomen and pelvis; if there is ascites or a dialysis catheter, sample that first.

Management

Resuscitate: the same for all three

Score NEWS2 (National Early Warning Score 2). Seven or more is high risk: urgent review by a clinician at FY2 level or above, blood gas with lactate, cultures, FBC, CRP, U&E, clotting.⁶
Balanced crystalloid such as Hartmann's: 250 mL over 10 to 15 minutes, reassess, repeat to a maximum of 1000 mL, then senior review. Oxygen to 94 to 98%.^6,7
Broad-spectrum intravenous antibiotics within 1 hour of calculating NEWS2. Keep nil by mouth. Refer to the senior surgical decision maker at the same time.^6,5

Secondary (surgical) peritonitis: antibiotics

Piperacillin with tazobactam 4.5 g intravenously every 8 hours alone, or gentamicin 5 to 7 mg/kg once daily with metronidazole 500 mg intravenously every 8 hours.^8,9,10,11
Colonic source: co-amoxiclav 1.2 g intravenously three times a day. Allergy to penicillins and cephalosporins: intravenous ciprofloxacin 400 mg two or three times daily plus metronidazole 500 mg three times a day.¹²
Review at 48 hours and step down to oral. Up to 14 days for a CT-confirmed abscess. Local antimicrobial policy overrides these choices.^12,7

Secondary peritonitis: source control

Antibiotics do not treat an uncontrolled source. Get the surgeons or interventional radiology involved now, not after the next set of blood tests.^5,2
Drain a diverticular abscess larger than 3 cm. Perforation with generalised peritonitis needs laparoscopic lavage or resection; faecal peritonitis found at operation means resection.¹²

Spontaneous bacterial peritonitis in cirrhosis

Tap the ascites on admission, without delay. Neutrophils above 250 cells/mm³ is the diagnosis. Inoculate blood culture bottles at the bedside.³
Cefotaxime is the most-studied antibiotic; the BNF dose for severe infection is 8 g daily intravenously in four divided doses. Local resistance decides the agent.^3,13
If creatinine is raised or rising: 20% human albumin solution 1.5 g/kg within 6 hours of diagnosis, then 1 g/kg on day 3.³
After the episode, ciprofloxacin 500 mg once daily by mouth or co-trimoxazole 960 mg once daily prevents recurrence. NICE advises against routine prophylaxis before a first episode.^3,14

Peritoneal dialysis peritonitis

Drain a bag and send effluent for cell count with differential, Gram stain and culture in blood culture bottles. Check the exit site. Telephone the renal unit.^4,1
Diagnosis: two of pain or cloudy effluent, white cells above 0.1 × 10⁹/L after a two-hour dwell with over half neutrophils, and a positive culture.⁴
Antibiotics go into the dialysate: vancomycin 15 to 30 mg/kg every 5 to 7 days with ceftazidime 1000 to 1500 mg daily. Treat for 14 days or longer.^4,8
Use the intravenous route only if the patient is systemically septic or intraperitoneal treatment would be delayed, then switch back to the dialysate.⁴

Not improving

Ascites: a neutrophil count that has not fallen below a quarter of the pre-treatment value at 48 hours means resistance or a missed surgical source. Re-image.³
Dialysis: effluent not clear after 5 days of appropriate antibiotics means catheter removal. Growth of several organisms suggests a perforated bowel.⁴
Surgical: persistent fever, ileus or organ failure after operation means an undrained collection or an anastomotic leak until CT proves otherwise.^5,2

Before discharge

Narrow the antibiotic once the source is confirmed or cultures return. Treat the cause: ulcer, diverticular, inflammatory bowel or cancer follow-up, liver clinic, or dialysis retraining.^7,1
Safety-net: return urgently for worsening pain, fever, vomiting, confusion, reduced urine output or cloudy dialysis fluid.¹

Exam traps

A soft, non-tender abdomen does not exclude peritonitis in an older, diabetic, immunosuppressed or steroid-treated patient; steroids blunt the signs you are examining for.
Not all peritonitis in ascites is spontaneous. Several organisms, a very high neutrophil count or localised signs mean a perforation, which needs CT and a surgeon.
A normal erect chest radiograph does not exclude perforation; it is far less sensitive than CT and must never delay it.
In peritoneal dialysis, growth of both Gram-positive and Gram-negative organisms suggests a perforated bowel, not a contaminated exchange.
NICE does not recommend routine antibiotic prophylaxis against a first episode of spontaneous bacterial peritonitis, only in severe liver disease.
Lactate can be normal in early peritonitis. Treat the abdomen you are examining, not the number.

Illustrations

Pelvic collection on CTCoronal contrast-enhanced CT showing a pelvic fluid collection as a potential source of secondary peritonitis and a target for image-guided drainage.Poudel A et al., Cureus 2026, CC-BY-4.0 · CC-BY-4.0

Key sources

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.