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Clinical Pathology

Examination of Peritoneal Fluid

Dayyal Dg.
By Dayyal Dg.
Published: Thursday, 01 November 2018
Updated: Wednesday, 05 December 2018 21:59
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Examination of Peritoneal Fluid
Examination of Peritoneal Fluid

The peritoneal cavity is a potential space in the abdomen lined by mesothelial cells and normally containing about 30-50 ml of serous fluid. The fluid is an ultrafiltrate of plasma and its formation is dependent upon hydrostatic pressure, plasma oncotic pressure, and capillary permeability.

Pathological accumulation of fluid in the peritoneal cavity is called as ascites, and the accumulated fluid is called as ascitic fluid.

Causes of Ascites

Causes of ascites are listed in Table 1186.1. Causes are classified based on whether the fluid is a transudate or an exudate. Unlike pleural fluid, there are no well-defined criteria for distinction between transudates and exudates. Majority of patients with ascites have cirrhosis of the liver; the presence of ascites in a patient with cirrhosis is a poor prognostic sign.

Table 1186.1 Causes of ascites
Transudate (increased hydrostatic pressure or plasma oncotic pressure) Exudate (increased capillary permeability or lymphatic obstruction)
1. Cirrhosis of liver 1. Bacterial peritonitis (primary or secondary)
2. Congestive cardiac failure 2. Tuberculosis
3. Hypoproteinemia 3. Malignancy (lymphoma, hepatoma, metastatic carcinoma)
4. Abdominal injury
5. Biliary peritonitis (rupture of the gallbladder)
6. Pancreatitis
7. Chylous ascites (obstruction of or injury to thoracic duct)

Indications for Abdominal Paracentesis

Abdominal paracentesis refers to the removal of ascitic fluid through puncture of the peritoneal cavity. Indications for abdominal paracentesis are given in Table 1186.2.

Table 1186.2 Indications for abdominal paracentesis
1. All patients with new-onset ascites
2. At admission in all patients with ascites for detection of asymptomatic infection
3. All patients with ascites who develop clinical features of bacterial infection, hepatic encephalopathy, gastrointestinal hemorrhage, or impairment of renal function.
4. Symptomatic ascites (therapeutic paracentesis)

Collection of Sample

Presence of ascites can usually be detected by clinical examination; if clinical examination is not definitive, ultrasound can be helpful. Ultrasonography can also be useful for determining the cause of ascites.

A hollow needle is inserted through the abdominal wall (usually left lower quadrant of the abdomen below the border of shifting dullness) into the peritoneal cavity (Figure 1186.1) and fluid (20-50 ml) is removed under aseptic precautions. For cytology, to maximize the yield of malignant cells, 100 ml should be submitted. For cell count, the sample is collected in EDTA-containing tube. For microbiologic culture, the sample is inoculated in blood culture bottles at the bedside.

Figure 1186.1 Sites for paracentesis blacked circles
Figure 1186.1 Sites for paracentesis (blacked circles)

Complications of the procedure include hemorrhage, perforation of viscus, and the introduction of infection. Evidence of fibrinolysis or of disseminated intravascular coagulation in liver disease is a contraindication for paracentesis.

Examination of Ascitic Fluid

Laboratory analysis of ascitic fluid helps in the differential diagnosis of ascites. A variety of tests can be carried out; however, the tests should be decided in an individual patient according to the clinical presentation. The commonly performed tests include estimation of total proteins and albumin, cell count, cytological examination, and bacterial culture.

Tests were commonly done on the ascitic fluid

  • White Cell Count: This is the most important test as it rapidly provides information about possible bacterial infection. Neutrophil count >250/cmm is a strong indication of bacterial infection, whereas lymphocytosis indicates peritoneal tuberculosis or carcinomatosis.
  • Albumin: Estimation of serum and ascitic fluid albumin allows calculation of serum-ascites albumin gradient (SAAG) that allows categorization of ascites into low and high SAAG.
  • Microbiological Tests: Gram stain, Ziehl-Neelsen stain, culture
  • Cytological Examination: For detection of malignant cells when peritoneum is involved by cancer.

1. Appearance

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

2. Chemical Examination

(A) Proteins:

Traditionally, fluid is called as a transudate if protein content is low, and an exudate if its protein content is high. However, this criterion alone is not always sufficient. In ascitic fluid, distinction between transudates and exudates cannot be reliably made by estimation of proteins. A better indicator is the albumin gradient (calculated as serum albumin minus ascitic fluid albumin done on the same day) (Figure 1186.2). 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).

Classification of ascites into high and low albumin gradient
Figure 1186.2 Classification of ascites into high and low albumin gradient. This system has replaced test for total protein concentration in ascitic fluid for classification of ascites into transudate and exudate. Patients with high albumin gradient respond to sodium restriction and diuretics, while patients with low albumin gradient require specific treatment

(B) Lactate Dehydrogenase:

Lactate dehydrogenase in ascitic fluid is 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. Ascitic fluid findings in various diseases are shown in Table 1186.3. The distinction between spontaneous and secondary bacterial peritonitis is presented in Table 1186.4 and Figure 1186.3.

Table 1186.3 Ascitic fluid findings in various diseases
Cause Appearance Type of fluid Cells Special studies
1. Spontaneous bacterial peritonitis Cloudy or turbid Exudate Neutrophils ≥250/μl Single organism isolated on culture; Proteins <1.0 gm/dl; Glucose normal
2. Secondary bacterial peritonitis Cloudy or turbid Exudate Neutrophils ≥1000/μl Multiple organisms isolated on culture
3. Cirrhosis of liver Clear, straw-colored Transudate Lymphocytes <500/μl Albumin gradient ≥1.1 gm/dl
4. Tuberculous peritonitis Serosanguineous Exudate Lymphocytes >500/μl Ziehl-Neelsen stain; culture; Low albumin gradient; Total proteins ≥2.5 gm/dl
5. Malignancy Bloody Exudate Lymphocytes, malignant cells Cytology
Table 1186.4 Distinction between spontaneous and secondary bacterial peritonitis
Parameter Spontaneous bacterial peritonitis Secondary bacterial peritonitis
1. Obvious source of infection Absent Present, e.g. perforation of viscus, abscess
2. Total ascitic fluid proteins <1.0 gm/dl ≥1.0 gm/dl
3. Severity Less severe More severe
4. Culture Single organism Multiple organisms
5. Treatment Rapid response to antibiotics Requires surgical treatment
Differentiation of spontaneous from secondary bacterial peritonitis
Figure 1186.3 Differentiation of spontaneous from secondary bacterial peritonitis

(C) Amylase:

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

(D) Bilirubin:

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

3. Cell Count

Cell count is usually done to distinguish cirrhotic ascites from spontaneous bacterial peritonitis. In ascitic fluid, total leukocyte count >500/ml and absolute neutrophil count >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 85% (as compared 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 helpful in diagnosis of tuberculous peritonitis.

5. Cytological Examination

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

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Examination of Peritoneal Fluid
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