What diagnostic tests should be performed on ascitic (ascites) fluid?

From the Guidelines

When evaluating ascites fluid, you should perform a diagnostic paracentesis with several key tests, prioritizing cell count with differential, albumin measurement to calculate serum-ascites albumin gradient (SAAG), and culture with sensitivity, as these are crucial for differentiating between portal hypertension, malignancy, infection, and other causes of ascites, as recommended by the most recent guidelines 1. The initial step in evaluating ascites fluid involves a gross examination, followed by key laboratory tests. These tests should include:

• Cell count with differential to assess for signs of infection or malignancy
• Albumin measurement to calculate the SAAG, which helps differentiate between portal hypertension and other causes of ascites
• Total protein and glucose levels to further characterize the ascites fluid
• Lactate dehydrogenase (LDH) to help differentiate between spontaneous and secondary bacterial peritonitis
• Culture with sensitivity to identify any infectious organisms and guide antibiotic therapy Additional tests may be considered based on clinical suspicion, such as:
• Cytology for malignant cells if malignancy is suspected
• Gram stain and pH measurement to further evaluate for infection
• Neutrophil count and bacterial cultures if spontaneous bacterial peritonitis is suspected
• Adenosine deaminase testing and acid-fast bacilli staining if tuberculosis is a concern
• Amylase and triglyceride levels to identify pancreatic ascites or chylous ascites, respectively The SAAG is a critical test, as values ≥1.1 g/dL suggest portal hypertension, while values <1.1 g/dL point to other etiologies like malignancy or infection 1. It's also important to note that the use of reagent strips for rapid diagnosis of spontaneous bacterial peritonitis is not recommended due to low sensitivity and high risk of false negative results 1. Prompt and accurate testing allows for appropriate diagnosis and treatment initiation, which is crucial for improving patient outcomes in terms of morbidity, mortality, and quality of life.

From the Research

Ascites Fluid Analysis

To determine the cause of ascites, several tests can be run on the ascites fluid. The following are some of the key tests:

• Cell count and differential, including polymorphonuclear neutrophils (PMN), lymphocytes, eosinophils, and macrophages 2, 3
• Serum-ascites albumin gradient (SAAG) to classify the cause of ascites into portal hypertension-related and non-related 4, 3, 5
• Total protein concentration 5
• Fluid culture to detect bacterial peritonitis 6
• Cytology to detect malignant cells 5
• Optional tests, such as:
  • Cholesterol 5
  • Tumor markers 5
  • Lactate dehydrogenase 5
  • Adenosine deaminase (ADA) 5
  • Triglyceride 5
  • Amylase 5
  • Glucose 5
  • Brain natriuretic peptide (BNP) 5

Diagnostic Algorithm

A diagnostic algorithm can be established based on the results of these tests to determine the etiology of ascites 5. For example:

• If the PMN count is ≥250 cells/mm3, spontaneous bacterial peritonitis (SBP) is highly suspected 2, 3
• If the SAAG is ≥1.1 g/dl, portal hypertension-related ascites is likely 4, 3, 5
• If the SAAG is <1.1 g/dl, non-portal hypertension-related ascites, such as malignant ascites, is likely 3, 5