How do you calculate absolute neutrophil count from peritoneal fluid cell count?

Calculating Neutrophils from Peritoneal Fluid Cell Count

To calculate absolute neutrophil count in peritoneal fluid, multiply the total white blood cell (WBC) count by the percentage of neutrophils in the differential. 1

Procedure for Calculating Peritoneal Fluid Neutrophils

1. Collection and Processing:

   • Perform diagnostic paracentesis to obtain peritoneal fluid
   • Centrifuge the fluid sample
   • Prepare a smear with Giemsa stain
   • Examine under optical microscope for cell counts 1

2. Calculation Method:

   • Formula: Absolute Neutrophil Count = Total WBC count × (% neutrophils ÷ 100)
   • Example: If total WBC count is 1000 cells/mm³ and neutrophils are 70% of cells:
     • Absolute neutrophil count = 1000 × 0.70 = 700 neutrophils/mm³

Laboratory Techniques

Manual Counting

• Historically preferred method for peritoneal fluid
• Uses optical microscopy after Giemsa staining
• Can be completed in less than 4 hours 1

Automated Counting

• Recent studies show excellent correlation with manual counts
• Provides rapid results even at low neutrophil levels
• May replace manual counting in many settings 1

Clinical Significance

• Diagnostic Threshold: ≥250 neutrophils/mm³ is diagnostic of spontaneous bacterial peritonitis (SBP) in cirrhotic patients 1
• Sensitivity vs. Specificity:
  • 250 neutrophils/mm³ cutoff provides greatest sensitivity
  • 500 neutrophils/mm³ cutoff provides greatest specificity 1

Important Considerations

• Ascitic fluid should be analyzed promptly after collection
• Initial analysis should include cell count and differential, total protein, and serum-ascites albumin gradient (SAAG) 1
• Despite negative cultures in up to 60% of cases, neutrophil count remains the diagnostic standard for SBP 1
• Reagent strips (dipsticks) for rapid neutrophil detection are not recommended due to low sensitivity 1

Pitfalls to Avoid

1. Delayed Processing: Process samples promptly to avoid cell degradation
2. Improper Collection Technique: Use proper aseptic technique during paracentesis
3. Relying on Reagent Strips: These have low diagnostic accuracy for SBP detection 1
4. Misinterpretation of Results: Culture-negative neutropenia (≥250 neutrophils/mm³ with negative culture) should still be treated as SBP 1
5. Failure to Distinguish Secondary Peritonitis: Consider additional testing (protein, LDH, glucose) when secondary bacterial peritonitis is suspected 1

By following this approach to calculating peritoneal fluid neutrophil counts, clinicians can accurately diagnose conditions like spontaneous bacterial peritonitis and initiate appropriate treatment promptly.