What is the interpretation of a peritoneal fluid analysis showing an elevated RBC count, abnormal white blood cell differential, and elevated protein and glucose levels?

Peritoneal Fluid Analysis Interpretation

Primary Interpretation: This is NOT spontaneous bacterial peritonitis (SBP) and the lymphocyte-predominant pattern strongly suggests tuberculous peritonitis or peritoneal carcinomatosis requiring further diagnostic workup.

Critical Cell Count Analysis

The absolute PMN count is approximately 170 cells/mm³ (calculated as 5,670 total WBC × 3% granulocytes), which is below the diagnostic threshold of 250 cells/mm³ for SBP 1. This definitively rules out bacterial peritonitis and empiric antibiotics should NOT be initiated 1.

The predominance of lymphocytes (66%) with a low total WBC count (5,670/mm³) argues strongly against bacterial peritonitis 1. This lymphocytic pattern is the key finding that redirects the diagnostic approach.

Differential Diagnosis Based on Fluid Characteristics

Most Likely: Tuberculous Peritonitis

Tuberculous peritonitis is characterized by lymphocytosis in ascitic fluid, with this patient's 66% lymphocyte predominance being highly suggestive 2, 1. The elevated protein (5.4 g/dL) further supports this diagnosis, as tuberculosis typically shows protein >2.5 g/dL 1.

Immediate next steps for tuberculous peritonitis workup:

• Order ascitic fluid adenosine deaminase (ADA) level - a threshold >27 U/L has 100% sensitivity and 93.3% specificity for tuberculous peritonitis in cirrhotic patients 2, 1
• Send fluid for AFB smear and mycobacterial culture - though sensitivity is only 0-86% for smear and 20-83% for culture 2, 1
• Consider PCR testing for mycobacteria - this is the most rapid and accurate method 2

Alternative: Peritoneal Carcinomatosis

Malignancy-related ascites also presents with lymphocytic predominance and would show a serum-ascites albumin gradient (SAAG) ≤1.1 g/dL 1.

To evaluate for malignancy:

• Calculate SAAG - requires simultaneous serum albumin measurement 1
• Send 50 mL of fresh warm fluid for cytology immediately - sensitivity is 82.8% on first sample, 93.3% with two samples 1
• Measure ascitic fluid CEA - levels >5 ng/mL suggest malignancy with high specificity 2

Parameters That Rule Out Other Diagnoses

Secondary Peritonitis: EXCLUDED

The fluid glucose of 100 mg/dL is well above the <50 mg/dL threshold that suggests secondary peritonitis from perforated viscus 2. The LDH of 88 U/L is below the serum LDH level (which would be elevated in secondary peritonitis) 2. These findings make gut perforation extremely unlikely 2.

Pancreatic Ascites: EXCLUDED

No amylase level was provided, but pancreatic ascites typically shows amylase >1,000 mg/dL 2. The clinical context and other parameters do not suggest this diagnosis.

Urinary Ascites: EXCLUDED

The fluid creatinine of 0.63 mg/dL is not elevated compared to expected serum levels, ruling out bladder or ureteral injury 2.

Additional Fluid Characteristics

• RBC count of 620/mm³ is mildly elevated but non-specific; can occur with traumatic tap or underlying malignancy 3
• Cholesterol 85 mg/dL and triglyceride 27 mg/dL are within normal ranges and do not suggest chylous ascites
• pH 8.1 is alkalotic but not diagnostically specific in this context

Critical Management Algorithm

1. Do NOT start empiric antibiotics - PMN count <250 cells/mm³ 1
2. Order ADA level on stored fluid immediately - most critical next test 2, 1
3. Send fluid for AFB smear, culture, and PCR - if ADA unavailable or patient at high risk 2, 1
4. Calculate SAAG with simultaneous serum albumin - differentiates portal hypertension from other causes 1
5. Send cytology if SAAG ≤1.1 g/dL or ADA <27 U/L - evaluates for malignancy 1
6. Consider laparoscopy with peritoneal biopsy - if diagnosis remains uncertain after above workup, this is the gold standard 2

Common Pitfalls to Avoid

Do not treat empirically as bacterial peritonitis - the lymphocyte predominance and low PMN count make this diagnosis untenable, and unnecessary antibiotics will delay correct diagnosis 1.

Do not rely solely on AFB smear - sensitivity is poor (0-86%), so negative smear does not exclude tuberculosis 2, 1.

Do not assume single etiology - approximately 5% of patients have two or more causes of ascites (e.g., cirrhosis plus tuberculosis or malignancy) 2.

Do not delay ADA testing in cirrhotic patients - while ADA can show lower sensitivity in cirrhosis, a cutoff of 27 U/L still maintains excellent diagnostic performance 2, 1.