The measurement of Adenosine Deaminase (ADA) activity in ascitic fluid (fluid accumulated in the abdominal cavity) is primarily used as a diagnostic marker for Tuberculous Peritonitis (TBP). 🔑 Key Role: Tuberculous Peritonitis (TBP) Diagnosis TBP Indicator: Elevated ADA levels in ascitic fluid are a strong indicator of an active cellular immune response against Mycobacterium tuberculosis (the bacteria causing TB) in the peritoneum. Lymphocyte Activity: ADA is highly concentrated in T-lymphocytes. TBP causes an influx and stimulation of these lymphocytes, leading to a significant increase in ADA release into the ascitic fluid. Diagnostic Value: It is a rapid, inexpensive, and minimally invasive test, especially valuable in areas where TB is endemic. It helps avoid or target more invasive procedures like laparoscopy and peritoneal biopsy.