Ascitic fluid analysis: Laboratory Examination of Peritoneal fluid

Indications for Abdominal Paracentesis

Abdominal paracentesis is a medical procedure for removal of ascitic fluid by puncturing the peritoneal cavity. This procedure is usually done in a doctor’s office or an outpatient clinic and in an expert’s hands, it is normally safe, although there is a small chance of infection, excessive bleeding or perforating a loop of bowel.
The indications are outlined as follows;

• All patients with new-onset ascites
• Symptomatic ascites (therapeutic paracentesis)
• At admission in all patients with ascites for detection of asymptomatic infection
• All patients with ascites who develop clinical features of bacterial infection, hepatic encephalopathy, gastrointestinal hemorrhage, or impairment of renal function.

Specimen collection for Ascitic fluid analysis

The condition is usually be detected by clinical examination; if clinical examination is not definitive, ultrasound can be used. Ultrasonography can also be important for determining the cause of ascites.

A hollow needle is inserted through the abdominal wall (normally left lower quadrant of abdomen below the border of shifting dullness) into the peritoneal cavity and fluid (20-50 ml) is tapped under aseptic precautions.

• For cytology studies, to increase the yield of malignant cells, 100 ml should be collected and submitted.
• For cell count sample is collected in EDTA-containing tube.
• For microbiologic culture and sensitivity, the sample is inoculated in blood culture bottles at bedside.

Possible complications of the procedure include;

• Haemorrhage,
• Perforation of viscus,
• Introduction of infection.
• Evidence of fibrinolysis or of disseminated intravascular coagulation (DIC) in liver disease is a contraindication for paracentesis.

Procedure for Ascitic fluid analysis

Ascitic fluid analysis helps in the differential diagnosis of ascites. A variety of tests are done; however, the tests should be decided in an individual patient according to the clinical presentation.

The commonly performed tests include;

• Measurement of total proteins and albumin,
• Cell count,
• Cytological examination,
• Bacterial culture

White cell count:

This is the most useful test as it rapidly gives information about possible bacterial infection. Neutrophil count of more than 250/cmm is a strong indication of bacterial infection, whereas lymphocytosis indicates peritoneal tuberculosis or carcinomatosis.

1. Physical appearance:

• Transudates appear pale yellow or straw colored and clear, while exudates are opaque or turbid.
• Turbid fluid is as a result of leucocytes, malignant cells, or proteins.
• Bloody or hemorrhagic fluid is because of traumatic tap, recent surgery, abdominal trauma, or malignancy. A traumatic tap shows gradual clearing of fluid during aspiration.
• Milky or chylous fluid is from obstruction of lymphatic duct due to inflammation or malignancy (lymphoma, carcinomatosis), or from abdominal injury.

2. Chemical ascitic fluid analysis:

Proteins:

If protein content is low, fluid is called as a transudate, and an exudate if its protein content is high but however, this criterion alone is not always enough. In Ascitic fluid analysis, distinction between transudates and exudates cannot be reliably made by estimation of proteins alone.

A better indicator is albumin gradient SAAG.  Total protein concentration in ascitic fluid can be helpful in differentiating spontaneous (total protein <1.0 gm/dl) from secondary bacterial peritonitis (total protein > 1.0 gm/dl).

Lactate dehydrogenase(LDH):

Lactate dehydrogenase in ascitic fluid is usually elevated in spontaneous bacterial peritonitis (i.e. there is no obvious source of infection), secondary bacterial peritonitis (i.e. identifiable source of infection is present), and in peritoneal carcinomatosis

Amylase:

Usually, amylase in ascitic fluid is similar to serum amylase. If ascitic fluid amylase is three times more than serum amylase, ascites is most likely to be due to pancreatic disease such as acute pancreatitis.

Bilirubin:

Ascitic fluid bilirubin more than 6.0 mg/ dl and ascitic fluid bilirubin/serum bilirubin ratio greater than 1.0 indicate perforation of biliary tract (biliary peritonitis). Ascitic fluid is bile-stained.

3. Cell count:

Cell count is usually carried out distinguish cirrhotic ascites from spontaneous bacterial peritonitis. In ascitic fluid, total leukocyte count  is more than 500/ml and absolute neutrophil count > more than 250/ml constitute the presumptive evidence of spontaneous bacterial peritonitis.

4. Microbiological examination:

Gram smear is positive in 25% cases of spontaneous bacterial peritonitis. If ascitic fluid is inoculated in blood-culture bottles at bedside, sensitivity of isolation rises to up to 85% (as related to conventional method of inoculation in broth and agar plates in laboratory).

In spontaneous bacterial peritonitis, a single organism is isolated, while secondary bacterial peritonitis is polymicrobial. In case of tuberculosis, Ziehl-Neelsen stain has sensitivity of 25-30%, while culture is positive in about 50% of cases. Laparoscopic biopsy is more important l in diagnosis of tuberculous peritonitis.

5. Cytological examination:

Cytological analysis of peritoneal fluid can detect 40-65% cases of malignant ascites.

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