| Diagnostic test* | Specimen collection and handling | Findings in spontaneous bacterial peritonitis (SBP) | Findings in secondary bacterial peritonitis | Comments |
| Cell count | - Inject 1 mL of ascitic fluid into a tube containing an anticoagulant solution.
| - Patients with SBP typically have an absolute neutrophil count ≥250 cells/microL.
| - Ascitic neutrophil count is usually very high (eg, >4000 cells/microL).
| - The absolute neutrophil count is calculated by multiplying the total white blood cell count by the percentage of neutrophils in the differential.
- If fluid is bloody, calculate corrected neutrophil count by subtracting one neutrophil from the absolute neutrophil count for every 250 red cells/microL.
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| Microbiology testing |
| Gram stain | - For Gram stain, send a few mL of ascitic fluid in a sterile urine container or tube containing no anticoagulant.
| - Gram stain has low sensitivity for detecting bacteria in SBP.
| - Gram stain may show large numbers of different bacterial forms.
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| Culture in blood culture bottles | - Send ascitic fluid for culture in blood culture bottles.
- Inject a minimum of 10 mL of ascitic fluid into each bottle.
- Inoculate a minimum of 2 bottles (ie, aerobic and anaerobic culture).
| - In the setting of a neutrophil count ≥250 cells/microL, positive fluid culture results confirm the diagnosis of SBP. Typically, one organism is isolated.
| - Ascitic fluid culture often shows polymicrobial infection.
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| Ascitic fluid chemistries | - To measure all chemistries, inject a total of several milliliters of ascitic fluid into a tube containing no anticoagulant.
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| Albumin | | - Most patients with SBP have portal hypertension (SAAG ≥1.1 g/dL).
| | - Calculate SAAG by subtracting ascitic fluid albumin from serum albumin.
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| Total protein | | - Protein concentration <1 g/dL indicates higher risk for SBP because of low concentration of opsonins in ascites.
- Protein concentration does not change during an episode of SBP.
| - High total protein concentration in ascitic fluid (>1 g/dL [10 g/L]).
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| Glucose | | - Glucose concentration generally remains above 50 mg/dL (2.8 mmol/L).
| - Glucose concentration is usually below 50 mg/dL (2.8 mmol/L).
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| Lactate dehydrogenase (LDH) | | - LDH is usually elevated.
- LDH is released from neutrophils that have lysed.
| | - In sterile ascites, LDH generally ranges from 23 to 63 units/L.
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| Amylase | | - Amylase in ascitic fluid in SBP is usually less than serum amylase.
| - Amylase concentration is increased in secondary peritonitis from intestinal perforation.
| - Amylase in ascitic fluid is increased in patients with pancreatitis and may be more than 5 times the serum value.
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| Bilirubin | | - Bilirubin concentration is typically low in SBP and less than serum bilirubin.
| - Elevated bilirubin concentration suggests perforated viscus.
- Bilirubin in ascitic fluid is higher than that of serum and >6 mg/dL (>103 micromol/L).
| - Measure bilirubin concentration when ascitic fluid is dark orange or brown.
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September 2025
