How should peritonitis be diagnosed in a patient undergoing peritoneal dialysis?

Diagnosis of Peritonitis in Peritoneal Dialysis Patients

Diagnose peritoneal dialysis-associated peritonitis when at least 2 of 3 criteria are present: cloudy peritoneal effluent, abdominal pain, and peritoneal fluid white blood cell count >100/μL with >50% neutrophils. 1

Diagnostic Criteria

The diagnosis requires meeting at least 2 of the following 3 cardinal features:

• Cloudy peritoneal effluent - This is the most common presenting sign and should prompt immediate evaluation 1, 2
• Abdominal pain or tenderness - Present in 74-95% of cases, though not always present 3, 4
• Peritoneal fluid analysis showing:
  • White blood cell count >100 cells/μL (or >100/mm³) 1
  • Neutrophil predominance >50% of total white blood cells 1

Critical Diagnostic Steps

Immediate Actions When Peritonitis is Suspected

• Obtain peritoneal fluid samples BEFORE starting antibiotics for cell count, differential, Gram stain, and culture 3, 2
• Send fluid for both aerobic and anaerobic bacterial cultures 2, 5
• Perform Gram stain of the peritoneal effluent to guide initial antibiotic selection 2, 6

Additional Clinical Features to Assess

• Fever >38.5°C occurs in only 38% of patients, so its absence does not exclude peritonitis 3
• Gastrointestinal symptoms including nausea, vomiting, diarrhea, or absence of defecation may be present 3, 7
• Systemic signs such as tachycardia (62.5% of cases) or hypotension suggesting sepsis 3

Timing of Diagnostic Testing

All diagnostic measurements must be performed when the patient is clinically stable and at least 1 month after resolution of any previous peritonitis episode. 8, 1 This is critical because:

• Peritonitis transiently changes peritoneal transport characteristics to a high transporter state 8, 9
• Recent peritonitis decreases ultrafiltration and can falsely alter clearance measurements 8, 9
• Testing too soon after peritonitis may overestimate or underestimate true peritoneal function 8

Microbiological Considerations

Common Causative Organisms

• Gram-positive cocci (most frequent): Coagulase-negative staphylococci, Staphylococcus epidermidis, and Staphylococcus aureus are the predominant pathogens worldwide 2, 6
• Gram-negative organisms are associated with higher rates of catheter loss and mortality 5, 6
• Fungal peritonitis (primarily Candida species) requires immediate catheter removal 2

Route of Infection Clues

• Coagulase-negative staphylococci suggest touch contamination during exchanges 6
• Pseudomonas aeruginosa or S. aureus most often indicate catheter-related infections 6
• Enteric organisms suggest bacterial translocation from the intestine, especially with concurrent diarrhea 1, 7

Important Caveats and Pitfalls

When Diagnosis is Challenging

• Not all three diagnostic criteria are always present - proceed with treatment if 2 of 3 criteria are met 1
• For unusual organisms that are difficult to identify with routine cultures, consider 16S rRNA gene sequencing 4
• Culture-negative peritonitis can occur; treat empirically based on clinical presentation 2, 6

Differential Diagnosis Considerations

• Evaluate for secondary peritonitis from gastrointestinal perforation if multiple organisms are isolated or if there is treatment failure 3
• Consider imaging (CT scan) if secondary peritonitis is suspected, as it has the highest sensitivity and specificity 3
• Assess for catheter-related mechanical problems that may present with cloudy effluent but without infection 9

Quality Monitoring

• Each dialysis unit should track peritonitis rates, causative organisms, and outcomes as part of continuous quality improvement 1, 6
• The target peritonitis rate should be <0.67 episodes per patient-year on dialysis 2
• Reassess residual kidney function after each peritonitis episode, as it can have significant negative impact 1