What lab tests should be performed on ascitic fluid obtained during an ascitic tap?

From the Guidelines

When performing an ascitic tap, the essential laboratory tests that should be conducted on the ascitic fluid include cell count with differential, albumin level, total protein, culture and sensitivity, and glucose level, as these tests provide valuable diagnostic information to guide treatment decisions for the underlying cause of ascites. The following tests are recommended:

• Cell count with differential to identify spontaneous bacterial peritonitis (SBP) if the neutrophil count exceeds 250 cells/mm³ 1
• Albumin level to calculate the serum-ascites albumin gradient (SAAG) and differentiate between portal hypertension-related ascites and non-portal hypertension causes
• Total protein to assess the risk of SBP
• Culture and sensitivity in blood culture bottles at the bedside for better yield to identify specific pathogens in infected ascites
• Glucose level, as decreased levels may be seen in SBP or malignancy Additional useful tests include lactate dehydrogenase (LDH), amylase, triglycerides, cytology, and acid-fast bacilli smear and culture when tuberculosis is suspected 1. It is also important to note that patients with ascitic fluid PMN counts greater than or equal to 250 cells/mm³ should receive empiric antibiotic therapy, and the ascitic fluid should be tested for total protein, LDH, glucose, and Gram’s stain to assist with the distinction of SBP from secondary peritonitis 1. The most recent and highest quality study 1 supports the use of these tests to guide diagnosis and management of ascites.

From the Research

Lab Tests for Ascitic Fluid

The following lab tests should be performed on ascitic fluid obtained during an ascitic tap:

• Polymorphonuclear (PMN) count: to diagnose spontaneous bacterial peritonitis (SBP) 2, 3, 4, 5, 6
• Bacteriological culture: to confirm the diagnosis of SBP and identify the causative bacteria 2, 3, 5, 6
• Reagent strip testing: such as Multistix or Combur(2) test, to rapidly diagnose SBP 3, 5
• D-dimer testing: to rule out SBP, with a cutoff value of 1500 ng/mL 4
• Heparin-binding protein (HBP) testing: although its diagnostic benefit is limited 4
• C-reactive protein (CRP), procalcitonin (PCT), and peripheral blood white blood cell (WBC) count: although their specificity and sensitivity are limited 4

Clinical Characteristics Associated with SBP

The following clinical characteristics are associated with an increased risk of SBP:

• History of jaundice 6
• Low arterial blood pressure on admission 6
• Encephalopathy 6
• Abdominal pain and temperature abnormalities, although their specificity and sensitivity are low 2