Paracentesis (performing an ascitic tap) is simple technique that can be used for diagnostic or therapeutic purposes

• if a tap is performed for diagnostic purposes then a small volume of ascitic fluid is required whereas when a therapeutic tap is performed then it may be necessary to withdraw several hundred milimitres of ascitic fluid
• the procedure should be performed by inserting a needle into the either iliac fossae under strict sterile conditions (thus avoiding the inferior epigastric vessels)
• outline of procedure:
  • therapeutic tap
    • patient should void bladder his/her before the procedure
    • the patient should lie supine or slightly over towards the side of aspiration
    • the clinician should tap out area of shifting dullness where there are no solid organs. The best sites for performing the ascitic tap are generally the iliac fossae
    • the skin overlying the site of the planned ascitic tap should be cleaned
    • local anaesthetic is inflitrated into the skin and deeper tissues at the site of the planned tap. The clinician performing the tap must aspirate for blood before each infiltration so that the local anaesthetic is not administered directly into the circulation
    • the cannula (or suprapubic catheter) is inserted and then the needle is withdrawn leaving the tubing in situ. The tubing is attached to a drainage bag. The ascitic fluid is then drained. Note that care must be taken not to remove too much fluid because profound hypotension may ensue
    • during the procedure the position of the patient may need to be adjusted in order to maintain flow
  • diagnostic tap
    • procedure is similar but amount of ascitic fluid drained is much smaller (approximately 50ml)
  • characteristics of ascitic fluid:
    • biochemical and cytological analysis of ascitic fluid provides important information
      • ascites in patients with cirrhosis was considered to have the characteristics of a transudate, with a total protein concentration of less than 2.5 g/dL and relatively few cells
        • serum-ascites albumin gradient
          • calculated via substracting the concentration of the ascites fluid albumin from serum albumin (1,2)
            • a serum–ascites albumin gradient of more than 1.1 g/dL predicts portal hypertension with great accuracy than the transudate/exudate classification
              • causes of a high serum-ascites albumin gradient include:
                • Cirrhosis
                • Congestive heart failure
                • Fulminant hepatic failure
                • Alcoholic hepatitis
                • Liver metastases
                • Veno-occlusive disease
                • Portal vein thrombosis
                • Budd–Chiari syndrome
              • causes of a low serum-ascites albumin gradient (of less than 1.1 g/dL) include:
                • Peritoneal carcinomatosis
                • Pancreatitis
                • Biliary pancreatitis
                • Peritoneal tuberculosis
                • Chlamydia/gonococcal infection
                • Nephrotic syndrome
                • Connective tissue diseases
        • ascitic fluid in cirrhosis generally contains fewer than 300–500 white blood cells/mm
          • of these white blood cells, more than 70% of these cells are mononuclear leucocytes
            • if ascitic fluid contains more than 250 neutrophils/mm3, a diagnosis of spontaneous bacterial peritonitis is made (3)
      • development of ascites is an important landmark in the natural history of cirrhosis and liver transplantation should be considered definitive treatment (4)

Reference:

1. Baccaro ME et al. Ascites. Medicine 2007; 35 ( 2): 104-107
2. Runyon BA et al. The serum-ascites albumin gradient is superior to the exudate-transudate concept in the differential diagnosis of ascites. Ann Intern Med 1992; 117: 215–220.
3. Rimola A et al. Diagnosis, treatment and prophylaxis of spontaneous bacterial peritonitis: a consensus document. International Ascites Club. J Hepatol 2000;32: 142–153.
4. Muhammed S et al. Portal hypertension and ascites. Surgery 2007 (Oxford); 25 (1): 28-33.