What labs are used to diagnose Spontaneous Bacterial Peritonitis (SBP)?

Diagnostic Labs for Spontaneous Bacterial Peritonitis (SBP)

The diagnosis of SBP is definitively established when the ascitic fluid neutrophil count exceeds 250 cells/mm³, which is the primary diagnostic laboratory test for SBP. 1

Primary Diagnostic Test

• Ascitic fluid neutrophil count > 250/mm³ (polymorphonuclear leukocytes)
  • Gold standard for diagnosis 2, 1
  • Must be performed via diagnostic paracentesis
  • Results should be obtained before initiating antibiotics, but treatment should not be delayed if SBP is suspected

Required Laboratory Tests for Paracentesis

1. Cell count with differential

   • Essential for diagnosis (neutrophil count > 250/mm³)
   • Should be performed by microscopy 2

2. Ascitic fluid culture

   • Should be inoculated at bedside into blood culture bottles 2
   • At least 10 mL of ascitic fluid should be used per bottle 2
   • Increases culture sensitivity to >90% 2, 1
   • Both aerobic and anaerobic bottles should be used 2
   • Culture is essential for guiding antibiotic therapy but not necessary for initial diagnosis 2

3. Blood cultures

   • Should be obtained simultaneously with paracentesis 2
   • Increases the possibility of isolating the causative organism 2

4. Total protein in ascitic fluid

   • Helps assess risk for SBP (low protein <1.5 g/dL increases risk) 1
   • Aids in differentiating SBP from secondary bacterial peritonitis 1

Additional Tests to Consider

For cases where secondary bacterial peritonitis is suspected:

• Ascitic fluid glucose (< 50 mg/dL suggests secondary peritonitis) 1
• Ascitic fluid LDH (higher than upper limit of normal for serum suggests secondary peritonitis) 1

Rapid Diagnostic Options

• Leukocyte esterase reagent strips (dipstick test)

  • Can be used for rapid bedside diagnosis if available 3, 4
  • With cutoff of 2+, sensitivity 80% and specificity 91% 4
  • Not yet recommended as replacement for cell count in guidelines 2

• Ascitic fluid lactoferrin

  • Research shows promise as a diagnostic marker (sensitivity 95.5%, specificity 97% at cutoff of 242 ng/mL) 5
  • Not yet incorporated into clinical guidelines

Common Pitfalls to Avoid

1. Delaying paracentesis - increases mortality by 10% for every hour's delay in initiating antibiotics in patients with septic shock 1

2. Improper culture technique - failure to inoculate blood culture bottles at bedside reduces culture sensitivity 2, 1

3. Missing asymptomatic SBP - up to one-third of patients may be asymptomatic 2, 1

4. Not performing paracentesis in all cirrhotic patients with ascites at hospital admission - guidelines recommend diagnostic paracentesis for all cirrhotic patients with ascites at admission, even without symptoms 2, 1

5. Withholding paracentesis due to coagulopathy concerns - routine transfusion before paracentesis is not recommended 3

When to Perform Diagnostic Paracentesis

• All patients with cirrhosis and ascites at hospital admission 2, 1
• Patients with signs/symptoms of infection 2
• Patients with gastrointestinal bleeding 2, 1
• Patients with shock or signs of systemic inflammation 2, 1
• Patients with worsening liver or renal function 2, 1
• Patients with hepatic encephalopathy 2, 1
• Patients with tense ascites and acute kidney injury 2

By following this diagnostic approach, you can promptly identify SBP and initiate appropriate antibiotic therapy to reduce morbidity and mortality in these high-risk patients.