Diagnostic paracentesis and ascitic fluid analysis

What it is

Ultrasonography is used to diagnose ascites, and paracentesis is done for newly discovered ascites, with calculation of the serum-ascites albumin gradient (SAAG) to diagnose the cause of the ascites. Paracentesis with an ascitic fluid granulocyte count and culture is done for any change in mental status or clinical condition, to diagnose spontaneous bacterial peritonitis.

How it is diagnosed

A high SAAG, 11.1 g/L or 1.1 g/dL and above, means the ascites is portal hypertension mediated. Within that group the ascitic protein separates two groups of cause: a protein below 25 g/L (2.5 g/dL) points to advanced liver dysfunction, that is cirrhosis, fulminant hepatic failure or massive liver metastases, while a protein of 25 g/L (2.5 g/dL) and above points to cardiac ascites from right-sided HF or constrictive pericarditis, or to post-sinusoidal obstruction such as IVC obstruction and Budd-Chiari syndrome.

A low SAAG, below 11.1 g/L or 1.1 g/dL, means disease of the peritoneum, and the fluid is exudative: primary infections, bacterial or tuberculous; peritoneal carcinomatosis; secondary to intra-abdominal sepsis such as pancreatitis or biliary leakage; and serositis in connective tissue diseases. The other low group is hypoalbuminemia, where the fluid is transudative, and it holds three: nephrotic syndrome, protein-losing enteropathy, and severe protein-energy malnutirion.

The same two numbers can be read as a grid. Ascitic fluid protein below 2.5 g/dL with SAAG above 1.1 is cirrhosis, and with SAAG below 1.1 is nephrotic syndrome. Protein above 2.5 g/dL with SAAG above 1.1 is right-sided HF or Budd-Chiari syndrome, and with SAAG below 1.1 is malignancy or TB. An ascitic fluid granulocyte count above 250/uL confirms spontaneous bacterial peritonitis.

When tuberculosis is suspected, fluid examination, whether cerebrospinal, ascitic, pleural, pericardial or joint, has a classically very low yield, so tissue biopsy from the affected site is more important, because there are generally fewer organisms.

Before any of those numbers comes the gross appearance of the fluid, which carries a diagnosis of its own. Clear fluid is the usual finding in cirrhosis, where it is translucent yellow. Turbid or cloudy fluid means infection. Milky fluid is chylous ascites, and it is defined by a triglyceride concentration that exceeds the serum concentration and is greater than 200 mg/dL (2.26 mmol/L). Pink or bloody fluid usually has a red cell concentration of more than 10,000 per mm3, from a traumatic tap or from malignancy.