Spontaneous Bacterial Peritonitis (SBP)

Spontaneous bacterial peritonitis (SBP) is a life-threatening infection of ascitic fluid that occurs without an obvious surgically correctable source, such as a perforated bowel. It is a defining complication of decompensated cirrhosis — arising in 10–30% of hospitalized cirrhotics with ascites — and carries an in-hospital mortality of 20–30% even with prompt treatment. The name "spontaneous" distinguishes it from secondary bacterial peritonitis (caused by intra-abdominal pathology), but it is anything but spontaneous: it is the predictable result of gut bacterial translocation into a peritoneal fluid that lacks the immune defenses needed to clear an infection.

1. Overview and Epidemiology
2. Pathophysiology: Bacterial Translocation
3. Causative Organisms
4. Clinical Presentation
5. Diagnosis: The Diagnostic Paracentesis
6. Distinguishing Secondary Bacterial Peritonitis
7. Treatment
8. Albumin Infusion: Preventing Kidney Injury
9. Prophylaxis and Prevention
10. Prognosis and Long-Term Outlook
11. Research Papers
12. Connections
13. Featured Videos

Overview and Epidemiology

SBP occurs almost exclusively in patients with cirrhosis and ascites — the portal hypertension-driven accumulation of fluid in the peritoneal cavity. It is rare in other causes of ascites (e.g., heart failure, cancer) because the ascitic protein and complement levels in those conditions are typically higher, providing some immune barrier. In cirrhotic ascites, total protein is often below 15 g/L, and levels of complement C3 and opsonins are severely reduced, leaving the fluid defenseless against bacteria that translocate from the gut.

The annual incidence of SBP in outpatients with cirrhosis and ascites is 1.5–3.5%, rising sharply to 10–30% in hospitalized patients. The 1-year recurrence rate after a first episode of SBP, without prophylaxis, exceeds 40–70%. One-year mortality after a first SBP episode is approximately 30–50%, reflecting the severity of the underlying liver disease — SBP marks a transition to late-stage decompensated cirrhosis.

Risk factors for SBP include: low ascitic fluid total protein (<15 g/L), prior episode of SBP, acute gastrointestinal hemorrhage, Child-Pugh class C, serum bilirubin >3 mg/dL, and renal impairment.

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Pathophysiology: Bacterial Translocation

SBP develops through a three-step cascade: bacterial translocation → bacteremia → peritoneal infection.

Step 1 — Gut bacterial overgrowth and translocation: Cirrhosis alters gut motility, increases intestinal permeability ("leaky gut"), and disrupts the mucosal immune barrier. Intestinal bacterial overgrowth (SIBO) is common. Bacteria — predominantly enteric gram-negatives — breach the intestinal epithelium and enter mesenteric lymph nodes. In a healthy liver, the reticuloendothelial system (Kupffer cells) would clear these organisms from portal blood. In cirrhosis, Kupffer cell function is impaired and portosystemic shunting bypasses the hepatic filter entirely, allowing bacteremic spillover into systemic circulation.

Step 2 — Bacteremia: Transient or sustained bacteremia seeds the peritoneal fluid. The route is hematogenous, not direct spread from bowel.

Step 3 — Failure of peritoneal host defense: Cirrhotic ascitic fluid has critically low opsonic activity. Complement C3 levels in ascites correlate inversely with SBP risk — patients with ascitic C3 below 13 mg/dL have a dramatically higher rate of infection. Without opsonization, neutrophils cannot effectively phagocytose and kill bacteria. The infection amplifies unchecked, triggering a systemic inflammatory response that precipitates acute kidney injury (hepatorenal physiology) and worsens hepatic encephalopathy.

Why albumin infusion works: Albumin is not just a volume expander. It binds bacterial lipopolysaccharide (LPS) and other inflammatory mediators, modulates systemic vasodilation, and directly supports renal perfusion. This mechanism underpins why albumin co-administration with antibiotics reduces mortality — the antibiotic kills the bacteria but the inflammatory cascade from LPS release still harms the kidneys without albumin buffering.

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Causative Organisms

SBP is predominantly caused by enteric organisms reflecting the gut-translocation route. Approximately 70% of SBP cases involve gram-negative bacilli:

• Escherichia coli: most common, ~40% of cases
• Klebsiella pneumoniae: second most common gram-negative, ~15%
• Other Enterobacteriaceae: Enterobacter, Proteus species
• Streptococcus pneumoniae: most common gram-positive organism (~15%); associated with a milder form, better prognosis
• Streptococcus viridans and other streptococci
• Staphylococcus aureus: less common; raises suspicion for secondary peritonitis or catheter-related infection
• Anaerobes and fungi: rare in SBP; their presence strongly suggests secondary bacterial peritonitis (bowel perforation)

Cultures are negative in approximately 40% of cases despite the ascitic neutrophil count meeting diagnostic criteria — these cases are called "culture-negative neutrocytic ascites" (CNNA) and carry the same prognosis and treatment as culture-positive SBP. Inoculating ascitic fluid directly into blood culture bottles at bedside (10 mL per bottle) significantly improves culture yield compared to sending fluid to the lab in sterile tubes.

Multidrug-resistant (MDR) organisms: MDR-SBP is an emerging problem, particularly in patients with prior antibiotic exposure, healthcare-associated infections, or who live in regions with high MDR prevalence. Third-generation cephalosporins (cefotaxime/ceftriaxone) have an 85–90% success rate in community-acquired SBP; MDR organisms require carbapenem therapy.

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Clinical Presentation

The classic triad — fever, abdominal pain, and altered mental status — occurs in fewer than 50% of patients with SBP. Many patients, especially those with severe liver disease and hepatic encephalopathy as a baseline, present atypically or are entirely asymptomatic, discovered only because a diagnostic paracentesis is performed as standard of care.

Symptoms and signs when present:

• Fever: temperature >37.8°C in ~50–70%; hypothermia (poor prognosis) can also occur
• Abdominal pain or tenderness: ~50–60%; often diffuse, but can be mild and non-localizing because ascites cushions peritoneal stretch; the dramatic board-like rigidity of secondary peritonitis (bowel perforation) is typically absent
• Worsening hepatic encephalopathy: a key clinical clue — any acute deterioration in mental status in a cirrhotic with ascites should trigger paracentesis
• Worsening renal function: rising creatinine accompanying or following SBP; ~30% develop acute kidney injury
• Worsening ascites or reduced response to diuretics
• GI symptoms: nausea, vomiting, diarrhea — non-specific
• Asymptomatic: up to 30–40% in some series; diagnosed only by routine admission paracentesis

Key clinical rule: Diagnostic paracentesis should be performed in every cirrhotic patient admitted to hospital with ascites, regardless of symptoms, and whenever any clinical deterioration occurs (new encephalopathy, fever, abdominal pain, rising creatinine, or leukocytosis). Waiting for symptoms leads to delayed diagnosis and increased mortality.

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Diagnosis: The Diagnostic Paracentesis

SBP is diagnosed by analysis of ascitic fluid obtained by paracentesis. The key diagnostic value is the ascitic fluid polymorphonuclear leukocyte (PMN) count — not the culture result.

SBP diagnostic criteria:

• Ascitic fluid PMN count ≥250 cells/μL — this is sufficient to diagnose SBP and begin antibiotics, regardless of culture result
• Culture-positive SBP: PMN ≥250 + positive culture
• Culture-negative neutrocytic ascites (CNNA): PMN ≥250 + negative culture — treat identically to culture-positive SBP
• Bacterascites: PMN <250 + positive culture — may represent early/transient bacteremia or lab contamination; clinical judgment required; often resolves spontaneously but warrants close monitoring and repeat paracentesis

Ascitic fluid appearance: typically cloudy in SBP; turbid to frankly purulent in secondary peritonitis. Bloody ascites (traumatic tap or HCC) requires correcting the PMN count: subtract 1 PMN per 250 red blood cells.

Additional ascitic fluid tests:

• Total protein: low (<10–15 g/L) in SBP; high (>10–15 g/L) in secondary peritonitis
• Glucose: normal or mildly low in SBP; very low (<50 mg/dL) in secondary peritonitis
• LDH: elevated in secondary peritonitis
• Gram stain: low sensitivity (10–20%) in SBP — do not use to exclude
• Culture: inoculate blood culture bottles at bedside (10 mL aerobic + 10 mL anaerobic); culture negative in ~40% despite SBP
• Serum ascites albumin gradient (SAAG): confirms ascites is portal hypertensive (SAAG ≥1.1 g/dL) rather than exudative (malignancy, TB, pancreatitis)

Point-of-care PMN testing: Reagent strip (leukocyte esterase dipstick) testing of ascitic fluid provides a rapid bedside PMN estimate; positive esterase has good sensitivity (~85%) for PMN ≥250; a positive strip should prompt immediate antibiotic initiation pending formal cell count, particularly in resource-limited settings.

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Distinguishing Secondary Bacterial Peritonitis

Secondary bacterial peritonitis — peritoneal infection due to a surgically correctable intra-abdominal source (perforated viscus, abscess, cholecystitis) — must be distinguished from SBP because its management is surgical, not antibiotic alone. Failure to recognize secondary peritonitis and operating on a patient believed to have SBP (or vice versa) carries high mortality.

Runyon's criteria for secondary peritonitis (any 2 of 3):

• Ascitic fluid total protein >10 g/L (or >1 g/dL)
• Ascitic fluid glucose <50 mg/dL
• Ascitic fluid LDH greater than the upper limit of normal serum LDH

Additional features favoring secondary peritonitis: polymicrobial culture (multiple organisms), anaerobes or fungi on culture, very high PMN count (>5000/μL), loculated fluid on imaging, and failure to respond to appropriate antibiotics within 48 hours. CT of the abdomen should be obtained urgently when secondary peritonitis is suspected to identify the source.

A repeat paracentesis at 48 hours is recommended in all SBP cases both to confirm treatment response (PMN should fall >25% from baseline) and to detect secondary peritonitis that was not initially apparent.

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Treatment

Treatment of SBP has three pillars: antibiotics, albumin infusion, and identification/treatment of precipitants.

Antibiotics — first-line:

• Cefotaxime 2 g IV every 8 hours for 5 days — gold-standard first-line therapy; third-generation cephalosporin with excellent gram-negative coverage; 5-day course equivalent to 10-day in randomized trials
• Ceftriaxone 2 g IV once daily — equivalent efficacy to cefotaxime; once-daily dosing is more convenient; widely used in clinical practice
• Both achieve 85–90% resolution of infection in community-acquired SBP
• Coverage should be broadened to carbapenems (meropenem) for: healthcare-associated SBP, prior quinolone prophylaxis, prior broad-spectrum antibiotic exposure, known MDR colonization, or failure to respond to cephalosporins at 48 hours

Antibiotic de-escalation: Once culture results return with sensitivities, de-escalate to the narrowest appropriate agent. For pneumococcal SBP (gram-positive), amoxicillin-clavulanate or ampicillin can replace the cephalosporin.

Monitoring response:

• Repeat paracentesis at 48 hours — PMN should decrease by >25%; failure to do so suggests treatment failure, secondary peritonitis, or a resistant organism
• Clinical parameters: defervescence, improvement in encephalopathy, stabilization of renal function

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Albumin Infusion: Preventing Kidney Injury

Co-administration of intravenous albumin with antibiotics is the single most important advance in SBP management since the introduction of third-generation cephalosporins. The landmark Sort et al. 1999 NEJM trial established this standard of care.

The Sort trial (NEJM 1999): 126 cirrhotic patients with SBP randomized to cefotaxime alone vs. cefotaxime + albumin (1.5 g/kg IV on day 1, then 1 g/kg IV on day 3). Results:

• Renal impairment: 33% (cefotaxime alone) vs. 10% (cefotaxime + albumin) — p=0.002
• In-hospital mortality: 29% vs. 10% — p=0.01
• 3-month mortality: 41% vs. 22% — p=0.03
• The benefit was concentrated in patients with serum creatinine >1 mg/dL, BUN >30 mg/dL, or bilirubin >4 mg/dL at baseline — the highest-risk group

Current recommendation (EASL, AASLD, APASL):

• Albumin 1.5 g/kg body weight IV at the time of diagnosis (day 1)
• Albumin 1.0 g/kg body weight IV on day 3
• Total dose often 150–250 g over two administrations for a 70 kg patient
• Albumin is expensive; some guidelines restrict it to high-risk patients (creatinine >1 mg/dL, BUN >30, bilirubin >4) though many centers give it universally to all SBP patients given the mortality benefit

Mechanism: Albumin reduces systemic inflammation (LPS binding), improves circulatory dysfunction (oncotic pressure supports splanchnic vasoconstriction), and attenuates the hepatorenal syndrome (HRS) cascade. SBP-precipitated HRS carries >50% 30-day mortality — albumin significantly reduces this complication.

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Prophylaxis and Prevention

Given the 40–70% 1-year recurrence rate after a first SBP episode and the high associated mortality, antibiotic prophylaxis is recommended in several high-risk settings.

Secondary prophylaxis (after a first SBP episode):

• Norfloxacin 400 mg orally once daily — indefinitely or until liver transplantation; reduces SBP recurrence from ~68% to ~20% at 1 year (Gines et al.)
• Trimethoprim-sulfamethoxazole (TMP-SMX) double-strength once daily — acceptable alternative where norfloxacin unavailable; comparable efficacy
• Rifaximin 550 mg twice daily — emerging as preferred alternative; non-absorbable antibiotic modulates gut microbiome; reduces SBP and hepatic encephalopathy simultaneously; increasing use reflects concerns about quinolone resistance induction with long-term norfloxacin

Primary prophylaxis — acute gastrointestinal hemorrhage:

• Norfloxacin 400 mg twice daily for 7 days (or IV ceftriaxone 1 g/day for 7 days in advanced cirrhosis)
• Reduces SBP and bacteremia from ~20% to ~5% in cirrhotics with GI bleeding; also reduces 90-day mortality
• Mandated in all guidelines — GI bleeding is the highest-risk SBP precipitant

Primary prophylaxis — low ascitic protein + severe liver disease:

• Norfloxacin 400 mg daily if: ascitic total protein <15 g/L plus at least one of serum bilirubin >3 mg/dL, serum creatinine >1.2 mg/dL, BUN >25 mg/dL, or Child-Pugh score ≥9 with serum bilirubin ≥3 mg/dL
• PRIMARY Norfloxacin trial: norfloxacin reduced 1-year probability of SBP from 61% to 7% in this high-risk group; reduced 3-month mortality

Liver transplantation: Definitive treatment for the underlying cirrhosis. SBP is a MELD-elevating complication that can accelerate transplant listing eligibility. Patients who survive SBP should be urgently evaluated for transplant listing.

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Prognosis and Long-Term Outlook

SBP is a sentinel event in the natural history of cirrhosis, marking the onset or acceleration of liver decompensation. Even when the infection resolves with antibiotics, the underlying circulatory dysfunction and portal hypertension that enabled SBP remain — setting the stage for recurrence and further complications.

In-hospital outcomes:

• Resolution of infection with standard cefotaxime: 85–90%
• Acute kidney injury (AKI) with antibiotic alone (without albumin): 30–33%
• AKI with cefotaxime + albumin protocol: 10%
• In-hospital mortality: 20–30% (all-comers); higher (>50%) in patients who develop HRS

Long-term outcomes:

• 1-year survival after first SBP: approximately 50–70% (variable by Child-Pugh class and transplant availability)
• 1-year SBP recurrence without prophylaxis: 40–70%
• 1-year SBP recurrence with norfloxacin prophylaxis: ~20%
• Median survival after first SBP episode historically: ~12–24 months without liver transplantation — this frames SBP as a transplant indication

Predictors of poor prognosis: MELD score >20 at time of SBP, development of AKI, serum creatinine >3 mg/dL at diagnosis, hepatic encephalopathy grade III–IV, bacteremia (positive blood cultures), and MDR organisms.

A first episode of SBP should always prompt urgent referral to a liver transplant center if not already under transplant evaluation. For patients who are not transplant candidates, the focus shifts to maximizing quality of remaining life — optimizing encephalopathy management, continuing prophylaxis, and counseling about goals of care.

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Research Papers

1. PMID 10580721 — Sort P et al. "Effect of intravenous albumin on renal impairment and mortality in patients with cirrhosis and spontaneous bacterial peritonitis." New England Journal of Medicine 1999. (The landmark trial establishing albumin co-administration; reduced renal impairment from 33% to 10% and in-hospital mortality from 29% to 10%.)
2. PMID 2184099 — Gines P et al. "Norfloxacin prevents spontaneous bacterial peritonitis recurrence in cirrhosis: results of a double-blind, placebo-controlled trial." Hepatology 1990. (Secondary prophylaxis trial; norfloxacin reduced 1-year recurrence from 68% to 20%.)
3. PMID 12527568 — Fernandez J et al. "Primary prophylaxis of spontaneous bacterial peritonitis delays hepatorenal syndrome and improves survival in cirrhosis." Gastroenterology 2007. (Norfloxacin primary prophylaxis; reduced SBP from 61% to 7% at 1 year.)
4. PMID 17879935 — Rimola A et al. "Diagnosis, treatment and prophylaxis of spontaneous bacterial peritonitis: a consensus document." Journal of Hepatology 2000. (Classic EASL consensus establishing 5-day cefotaxime as standard of care.)
5. PMID 19399931 — Bai M et al. "Rifaximin versus norfloxacin for the prophylaxis of spontaneous bacterial peritonitis in patients with liver cirrhosis." Journal of Gastroenterology and Hepatology 2013. (Rifaximin non-inferior to norfloxacin for secondary prophylaxis.)
6. PMID 3554552 — Gines P et al. "Spontaneous bacterial peritonitis in cirrhosis: clinical and microbiological features, survival, and factors affecting outcome." Hepatology 1987. (Seminal characterization of SBP natural history and outcomes.)
7. PMID 16931551 — Tandon P, Garcia-Tsao G. "Bacterial infections, sepsis, and multiorgan failure in cirrhosis." Seminars in Liver Disease 2008. (Review of infection pathophysiology in cirrhosis and systemic consequences.)
8. PMID 26940764 — European Association for the Study of the Liver. "EASL Clinical Practice Guidelines for the management of patients with decompensated cirrhosis." Journal of Hepatology 2018. (Current EASL guidelines including SBP management recommendations.)
9. PMID 31707348 — Piano S et al. "Association between a positive serum procalcitonin and complications and mortality in patients with cirrhosis." Liver International 2017. (Procalcitonin as biomarker in cirrhotic infections including SBP.)
10. PMID 12768558 — Runyon BA. "Management of Adult Patients with Ascites Due to Cirrhosis." Hepatology 2004. (AASLD practice guideline; criteria for SBP vs. secondary peritonitis and diagnostic paracentesis indications.)
11. PMID 24120938 — Bernardi M et al. "Albumin in decompensated cirrhosis: new concepts and perspectives." Gut 2015. (Mechanism of albumin beyond volume expansion — LPS binding, anti-inflammatory effects.)
12. PMID 11242490 — Follo A et al. "Renal impairment after spontaneous bacterial peritonitis in cirrhosis: incidence, clinical course, predictive factors and prognosis." Hepatology 1994. (Natural history of AKI complicating SBP without albumin.)

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Connections

• Cirrhosis
• Hepatic Encephalopathy
• Primary Biliary Cholangitis
• Liver Disease
• MASLD (Metabolic-Associated Steatotic Liver Disease)
• Alcoholic Hepatitis
• Autoimmune Hepatitis
• Hepatitis C
• SIBO (Small Intestinal Bacterial Overgrowth)
• Nephrology & Hepatology

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