Spontaneous bacterial peritonitis (SBP)

Definition

Spontaneous bacterial peritonitis (SBP) is a bacterial infection of pre-existing ascites without an identifiable intra-abdominal source of infection such as perforation, cholecystitis or diverticulitis. It occurs mainly in ascites due to liver cirrhosis.

Diagnostically, SBP is defined by more than 250 neutrophils per µL of ascitic fluid, regardless of the culture result. It is distinguished from bacterascites: bacteria grown in culture without a raised neutrophil count.

Occurrence & epidemiology

SBP is the most common spontaneous bacterial infection in liver cirrhosis and accounts for about a quarter of all bacterial infections in inpatients with cirrhosis. In asymptomatic outpatients with ascites, the prevalence is at most about 3.5%, and in inpatients 8–36%.

Aetiopathogenesis

Usually a single organizm is involved. Gram-negative gut bacteria such as Escherichia coli and Klebsiella pneumoniae are common, as are gram-positive organizms such as Streptococcus pneumoniae, Staphylococcus aureus and enterococci. In community-acquired SBP gram-negative organizms predominate, in hospital-acquired SBP gram-positive organizms.

Risk factors for SBP:

• previous episode of SBP
• gastrointestinal bleeding
• low total protein in ascitic fluid (below 1.5 g/dL)
• bilirubin above 3.2 mg/dL and platelets below 98,000/µL
• high MELD score
• genetic factors such as variants in the NOD2 gene

Clinical features

Symptoms are often mild. Typical features are fever, malaise and diffuse, constant, mild to moderate abdominal pain. Peritoneal signs such as tenderness and rebound are present but blunted by the ascites.

Often only hepatic encephalopathy, worsening liver function, acute kidney injury or otherwise unexplained clinical deterioration are noticed; SBP may also be completely asymptomatic.

Diagnosis

Paracentesis

The key test is diagnostic paracentesis with cell count and differential. A neutrophil count above 250/µL establishes the diagnosis. In hemorrhagic ascites, the neutrophil count is reduced by 1 per 250 red cells/µL.

The fluid is placed at the bedside into aerobic and anaerobic blood culture bottles, at least 10 mL per bottle; blood cultures are taken as well. Even so, an organizm is identified in only about 36–59% of cases. Urine test strips alone are not sensitive enough.

Distinguishing secondary peritonitis

Pointers to secondary peritonitis, for example due to perforation or abscess, are:

• isolation of several organizms, including anaerobes or fungi
• new, marked abdominal symptoms
• markedly raised cell count or raised total protein in the ascitic fluid
• ascitic glucose below 2.7 mmol/L and ascitic LDH above the upper limit of normal for serum
• raised CEA (above 5 ng/mL) or raised alkaline phosphatase (above 240 U/L) in the ascitic fluid

If this is suspected, cross-sectional imaging is additionally performed to look for the source of infection.

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Further reading (open access)

1. MSD Manual Professional: Spontaneous Bacterial Peritonitis (SBP)
2. AWMF-Leitlinienregister 021-017: Komplikationen der Leberzirrhose (S2k-Leitlinie DGVS, Fassung 2019)
3. MSD Manual Professional: Ascites

Cross-references