What is the approach to fluid analysis, particularly for peritoneal or ovarian fluid, in patients presenting with abdominal pain or suspected peritoneal disease?

Approach to Peritoneal Fluid Analysis

For patients with suspected peritoneal disease or abdominal pain, perform diagnostic paracentesis immediately and obtain ascitic fluid for cell count with differential, total protein, albumin (to calculate SAAG), and bedside culture in blood culture bottles if infection is suspected. 1

Initial Fluid Collection and Handling

• Perform paracentesis on all hospitalized patients with new-onset ascites and any patient with ascites who develops fever, abdominal pain/tenderness, encephalopathy, renal failure, acidosis, or peripheral leukocytosis. 1
• Coagulopathy is rarely a contraindication—only clinically evident fibrinolysis or disseminated intravascular coagulation (occurring in <1 per 1,000 procedures) should preclude the procedure. 1
• Prophylactic fresh frozen plasma or platelets before paracentesis are not recommended, as bleeding complications are sufficiently uncommon. 1
• If infection is suspected, inoculate blood culture bottles with ascitic fluid at the bedside to maximize culture yield (though cultures remain negative in up to 60% of cases with clinical infection). 1

Essential Initial Tests

Cell Count and Differential

• Obtain absolute polymorphonuclear neutrophil (PMN) count immediately—this is the single most critical test for detecting spontaneous bacterial peritonitis (SBP). 1
• PMN count ≥250 cells/mm³ (0.25 × 10⁹/L) indicates infection and mandates empiric antibiotic therapy with intravenous cefotaxime 2g every 8 hours. 1
• Automated cell counting is accurate and faster than manual counts, making it acceptable for clinical use. 1
• Reagent strips (urine dipsticks) for leukocyte esterase are NOT recommended due to low sensitivity and high false-negative rates, particularly at low neutrophil counts. 1

Serum-Ascites Albumin Gradient (SAAG)

• Calculate SAAG by subtracting ascitic fluid albumin from serum albumin (obtained same day). 1
• SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy. 1
• This classification system is superior to the outdated exudate/transudate concept. 1

Total Protein

• Measure ascitic fluid total protein as part of initial screening. 1
• Low protein (<1 g/dL) with typical findings suggests uncomplicated cirrhotic ascites. 1
• If PMN count is elevated, also measure lactate dehydrogenase (LDH) and glucose to distinguish spontaneous from secondary bacterial peritonitis. 1

Additional Testing Based on Clinical Context

When Secondary Peritonitis is Suspected

• Order ascitic fluid LDH, glucose, total protein, and Gram stain when PMN ≥250 cells/mm³ to differentiate secondary from spontaneous peritonitis. 1
• Secondary peritonitis indicators: total protein >1 g/dL, LDH greater than upper limit of normal for serum, glucose <50 mg/dL, multiple organisms on culture. 1
• Ascitic fluid carcinoembryonic antigen >5 ng/mL or alkaline phosphatase >240 units/L suggests gut perforation. 1
• If secondary peritonitis is confirmed, add anaerobic coverage to third-generation cephalosporin and pursue surgical evaluation. 1

When Tuberculosis is Suspected

• Measure adenosine deaminase (ADA) levels in peritoneal fluid—sensitivity and specificity are 100% and 97% respectively using thresholds of 36-40 U/L. 1
• Consider measuring free interferon-gamma (IFN-γ) levels, which demonstrate 93% sensitivity and 99% specificity for peritoneal tuberculosis. 1
• These biomarkers provide supportive evidence but must be interpreted within the complete clinical context. 1

In Complicated Intra-Abdominal Infections (Non-Cirrhotic)

• Obtain intra-abdominal fluid cultures in patients with complicated intra-abdominal infections, as this is associated with improved survival (adjusted OR of mortality 0.85,95% CI 0.77-0.95). 1
• Culture results drive antimicrobial changes in approximately 8% of complicated appendicitis cases and enable escalation when resistant organisms are identified. 1
• Candida isolation from peritoneal fluid is associated with higher mortality (OR 2.15) and should prompt consideration of antifungal therapy. 1

Follow-Up Paracentesis

• Routine follow-up paracentesis is unnecessary in typical SBP cases (advanced cirrhosis, typical symptoms, single organism, dramatic clinical response). 1
• Repeat paracentesis if the clinical setting, symptoms, fluid analysis, organism(s), or treatment response are atypical, as this may indicate secondary peritonitis requiring surgical intervention. 1
• Repeat paracentesis is also indicated if new signs of infection develop during hospitalization. 1

Critical Pitfalls to Avoid

• Do not delay empiric antibiotics while awaiting culture results when PMN ≥250 cells/mm³—mortality increases with treatment delays. 1
• Patients with PMN <250 cells/mm³ but clinical signs of infection (fever >100°F, abdominal pain/tenderness) should also receive empiric antibiotics. 1
• Do not assume negative cultures rule out infection—60% of clinically infected ascites remains culture-negative despite elevated PMN counts. 1
• Avoid ordering extensive test panels on all specimens—use an algorithmic approach based on pretest probability. 1
• Blood contamination during paracentesis can confound interpretation; distinguish this from true hemoperitoneum or strangulating lesions by clinical context. 2