How should I perform and interpret a diagnostic paracentesis for ascites, including SAAG calculation, PMN count thresholds, and initial treatment of spontaneous bacterial peritonitis?

Ascitic Fluid Analysis: Performance and Interpretation

Perform diagnostic paracentesis immediately in all patients with new-onset grade 2 or 3 ascites or any hospitalized cirrhotic patient, even without symptoms, and diagnose spontaneous bacterial peritonitis when the ascitic fluid polymorphonuclear (PMN) count exceeds 250 cells/µL, initiating empiric antibiotics without delay. 1

When to Perform Diagnostic Paracentesis

Indications requiring immediate paracentesis:

• All patients with new-onset grade 2 (moderate) or grade 3 (large/tense) ascites 1
• Any cirrhotic patient with ascites admitted to hospital for any reason, even if asymptomatic 1, 2
• Patients with fever, abdominal pain, encephalopathy, gastrointestinal bleeding, shock, or acute kidney injury 1, 3
• Worsening of pre-existing ascites requiring hospitalization 1

Critical pitfall: Up to one-third of patients with spontaneous bacterial peritonitis are completely asymptomatic, making empiric paracentesis essential in all hospitalized cirrhotic patients with ascites. 1

Paracentesis Technique

Procedural steps:

• Insert needle 15 cm lateral to the umbilicus in the lower quadrants to avoid epigastric vessels 4
• Do not delay paracentesis to correct coagulopathy unless clinically evident disseminated intravascular coagulation or hyperfibrinolysis is present—serious hemorrhage occurs in only 0.2-2.2% of procedures 2
• Obtain at least 10 mL of ascitic fluid 1
• Inoculate blood culture bottles (both aerobic and anaerobic) at the bedside immediately before any antibiotics are administered—this increases culture sensitivity to >90% 1, 5
• Obtain simultaneous peripheral blood cultures to increase organism isolation rates 1

Essential Ascitic Fluid Tests

Mandatory tests for every diagnostic paracentesis:

• Neutrophil (PMN) count (manual or automated) 1
• Total protein concentration 1
• Albumin concentration (ascitic fluid) 1
• Culture (bedside inoculation into blood culture bottles) 1
• Simultaneous serum albumin to calculate SAAG 1, 2

Conditional tests based on clinical context:

• Cytology when malignancy is suspected 1
• Adenosine deaminase when tuberculosis is considered (levels <40 IU/mL exclude TB with 98% accuracy) 1
• Amylase when pancreatic ascites is suspected (typically >1000 IU/L or >6× serum amylase) 1
• BNP or NT-proBNP when cardiac ascites is suspected 2

Interpretation: Diagnosing Spontaneous Bacterial Peritonitis

Diagnostic threshold:

• PMN count >250 cells/µL establishes the diagnosis of SBP and mandates immediate empiric antibiotic therapy 1
• This threshold was chosen for maximum sensitivity to avoid missing cases, as untreated SBP carries significant mortality 1

Culture-negative neutrocytic ascites:

• PMN count >250 cells/µL with negative cultures represents the same disease process as culture-positive SBP 1
• These patients have similar morbidity and mortality and require identical treatment 1

Bacterascites:

• Positive culture with PMN count <250 cells/µL may represent transient colonization or early SBP 1
• Repeat paracentesis and treat if symptomatic or if the organism suggests true infection rather than contamination 1

Automated cell counters:

• Validated alternative to manual counting with 94-100% sensitivity and 100% specificity for diagnosing SBP 6, 7
• Total nucleated cell count <1000 cells/µL has 95.5% negative predictive value for excluding SBP 8

SAAG Calculation and Interpretation

How to calculate:

• SAAG = (serum albumin) - (ascitic fluid albumin) 1, 2
• Use albumin values obtained on the same day 1

Interpretation:

• SAAG ≥1.1 g/dL indicates portal hypertension with 97% accuracy (cirrhosis, cardiac failure, massive liver metastases) 1, 2, 5
• SAAG <1.1 g/dL effectively excludes portal hypertension and suggests peritoneal carcinomatosis, tuberculosis, or pancreatic disease 1, 2
• Calculate SAAG when the cause of ascites is not immediately evident or when non-cirrhotic causes are suspected 1

Important caveat: Approximately 5% of patients have two or more causes of ascites simultaneously (e.g., cirrhosis plus heart failure), so consider mixed etiologies when clinical features don't align 2

Ascitic Fluid Protein and SBP Risk

Risk stratification:

• Total protein concentration <1.5 g/dL (or <15 g/L) identifies patients at increased risk for developing SBP 1
• This threshold guides decisions about primary SBP prophylaxis 1
• Conflicting data exist on the exact predictive value, but the threshold remains widely used 1

Initial Treatment of Spontaneous Bacterial Peritonitis

Immediate empiric antibiotic therapy:

• Start IV antibiotics immediately after diagnosis without waiting for culture results—every hour of delay in septic shock increases mortality by 10% 1
• Community-acquired SBP: Cefotaxime 2 g IV every 12 hours or ceftriaxone 1, 3
• Nosocomial or healthcare-associated SBP: Use broad-spectrum coverage (carbapenem or piperacillin-tazobactam) based on local resistance patterns, as multidrug-resistant organisms represent 35% of infections in these settings 1, 3

Albumin infusion:

• Administer IV albumin 1.5 g/kg within 6 hours of diagnosis and 1.0 g/kg on day 3 1, 3
• Particularly critical in patients with serum creatinine ≥1.0 mg/dL or bilirubin ≥5 mg/dL 3
• This reduces mortality from 29% to 10% in controlled trials 1

Treatment duration and monitoring:

• Continue antibiotics for 5-7 days total 3
• Repeat diagnostic paracentesis at 48 hours to assess treatment response 3
• Treatment failure is defined as <25% decrease in PMN count from baseline, which should prompt broadening antibiotic coverage and CT imaging to exclude secondary bacterial peritonitis 3

Antibiotic stewardship:

• Narrow or de-escalate antibiotic coverage as soon as culture and susceptibility results are available 1
• Most spontaneous infections are monobacterial, with ~60% gram-negative bacteria (predominantly E. coli and Klebsiella pneumoniae) 1
• Recent shift toward gram-positive organisms and multidrug-resistant pathogens, especially in nosocomial cases 1

Critical Pitfalls to Avoid

• Never delay paracentesis in hospitalized cirrhotic patients even if asymptomatic—SBP can be silent in one-third of cases 1, 2
• Never skip bedside inoculation of blood culture bottles—delayed inoculation reduces culture yield from 77% to 100% 5
• Never wait for culture results before starting antibiotics when PMN count >250 cells/µL—inappropriate initial therapy in septic shock increases mortality tenfold 1
• Never assume cirrhosis is the only cause of ascites—always calculate SAAG to exclude malignancy, tuberculosis, or cardiac failure 2
• Do not withhold paracentesis for coagulopathy or thrombocytopenia unless DIC is present 2, 4