Ascitic Fluid Analysis

Ascites is free fluid in the peritoneal cavity. The commonest cause is portal hypertension from cirrhosis, but infection, malignancy, heart failure, nephrotic syndrome, pancreatitis and tuberculosis also cause it. Diagnostic paracentesis, a sample of 20 to 60 mL from the left or right lower quadrant under ultrasound, gives the cause and detects spontaneous bacterial peritonitis (SBP). It is a basic, safe test: serious complications occur in under 1% of patients.

The test is used in all patients with new onset ascites, in all patients with cirrhosis and ascites admitted to hospital, and in those with deterioration, fever, abdominal pain, encephalopathy, gastrointestinal bleeding or kidney injury. Delay in diagnosing SBP increases mortality.

Cirrhosis accounts for about 75 to 80% of ascites. About 60% of patients with compensated cirrhosis develop ascites within 10 years, and 1-year survival after onset is about 85%, falling to 50% at 2 years if refractory. SBP occurs in 10 to 30% of hospitalised cirrhotic patients with ascites and carries an in hospital mortality of 10 to 30% even with treatment.

Indications. New ascites of unknown cause, ascites in a patient with cirrhosis admitted to hospital, fever or abdominal pain or tenderness, encephalopathy, hypotension, gastrointestinal bleeding, worsening kidney function, suspected malignant or tuberculous ascites, and before starting therapy for ascites in a patient not previously assessed.

Specimen. Fluid collected aseptically into: EDTA tube for cell count and differential, sterile tube for chemistry (albumin, total protein, glucose, LDH, amylase, triglycerides), blood culture bottles inoculated at the bedside (10 mL into each of aerobic and anaerobic bottles, increasing culture yield from about 50% to over 80%), and a larger volume for cytology (at least 50 mL, with 100 mL better) and mycobacterial tests when indicated. Draw a serum albumin on the same day.

Appearance. Clear and straw coloured in cirrhosis. Turbid: infection. Bloody: malignancy, traumatic tap, tuberculosis, pancreatitis (red cells over 50 000 per microL). Milky: chylous ascites (triglycerides over 2.26 mmol/L, or 200 mg/dL), from lymphatic obstruction, lymphoma, cirrhosis, trauma or surgery. Green or brown: bile or bowel perforation.

Serum ascites albumin gradient (SAAG). Serum albumin minus ascitic albumin.

• SAAG 11 g/L (1.1 g/dL) or more: portal hypertension (97% accuracy). Causes: cirrhosis, alcoholic hepatitis, heart failure, Budd-Chiari syndrome, portal vein thrombosis, sinusoidal obstruction syndrome, massive liver metastases, and myxoedema.
• SAAG under 11 g/L: non-portal hypertension. Causes: peritoneal carcinomatosis, tuberculous peritonitis, pancreatic ascites, nephrotic syndrome, serositis, bowel obstruction or infarction, and biliary ascites.

Ascitic total protein. Under 25 g/L in cirrhosis suggests a higher SBP risk (under 15 g/L particularly), while over 25 g/L with high SAAG suggests heart failure or Budd-Chiari syndrome. Used with SAAG, it separates cardiac (protein over 25 g/L) from cirrhotic ascites.

Cell count.

• Polymorphonuclear (neutrophil) count of 250 per mm3 or more: spontaneous bacterial peritonitis, even with a negative culture. Treatment is started on the count alone.
• Count under 250 per mm3 with a positive culture: bacterascites, which is repeated or treated if symptomatic.
• Bloody fluid: subtract 1 neutrophil per 250 red cells.
• Lymphocytes predominate in tuberculosis, malignancy and chylous ascites.
• Secondary bacterial peritonitis (perforation, abscess): neutrophils usually over 250, with at least two of: glucose under 2.8 mmol/L (50 mg/dL), protein over 10 g/L and LDH over normal serum upper limit, and polymicrobial culture.

Other tests.

• Culture: single organism (Escherichia coli, Klebsiella, Streptococcus pneumoniae and enterococci) in SBP. Multiple organisms indicate perforation.
• Glucose and LDH: low glucose and high LDH suggest secondary peritonitis or malignancy.
• Amylase over 100 U/L, or ascitic to serum ratio over 5: pancreatic ascites.
• Triglycerides: over 2.26 mmol/L (200 mg/dL): chylous.
• Bilirubin: ascitic bilirubin over serum bilirubin suggests bile leak.
• Creatinine: ascitic to serum ratio over 1 suggests urine ascites after bladder injury.
• Adenosine deaminase over 30 to 40 U/L: tuberculous peritonitis (sensitivity 90%, specificity 90%), lower in cirrhosis.
• Cytology: sensitivity about 97% in peritoneal carcinomatosis (with 3 samples), and low in massive liver metastases or hepatocellular carcinoma with no peritoneal spread.
• CEA over 5 microg/L and CA 125 are not specific, and CA 125 is raised in cirrhosis and tuberculosis.
• NT-proBNP in ascitic fluid supports cardiac ascites.

Critical values and findings needing immediate action.

• Neutrophils 250 per mm3 or more: start empirical antibiotics (a third generation cephalosporin) at once, with intravenous albumin (1.5 g/kg on day 1, 1 g/kg on day 3) when creatinine is raised, bilirubin over 68 micromol/L (4 mg/dL) or other high risk markers.
• Cloudy or purulent fluid, polymicrobial growth or very high neutrophil count: secondary peritonitis, and urgent imaging and surgery.
• Bloody ascites with haemodynamic change after paracentesis.
• Ascites with hepatic encephalopathy, kidney injury or gastrointestinal bleeding: infection screen.
• Malignant cells: oncology referral.
• Ascitic protein under 15 g/L with Child-Pugh score 9 or more, or bilirubin 51 micromol/L (3 mg/dL) or more: prophylactic antibiotics.

Interferences and pitfalls.

• Never withhold paracentesis for coagulopathy (INR or platelets), since bleeding complications are rare, and prophylactic plasma or platelets are not needed.
• Delay in processing allows cell lysis. The cell count should be done within hours.
• Inoculating blood culture bottles at the bedside is essential. Culture in a sterile pot reduces yield.
• Diuretics concentrate the fluid and raise protein, which may mislead.
• Mixed ascites (cirrhosis with a second cause such as cancer or tuberculosis) gives an intermediate SAAG.
• Lymphocytic ascites with low SAAG should prompt tuberculosis testing, since smear and culture are often negative and laparoscopy with biopsy is more sensitive.
• A single negative cytology does not exclude malignancy.
• SAAG is unreliable if serum albumin is very low, and in the presence of hyperglobulinaemia, with a possible false high gradient.
• Fluid should be examined before antibiotics are started when possible, without delaying them.

Special groups.

• Children: causes include liver disease, nephrotic syndrome, cardiac disease and malignancy, with chylous ascites after surgery.
• Pregnancy: uncommon, and causes such as portal hypertension, Budd-Chiari syndrome and ovarian hyperstimulation are considered.
• Older adults with heart failure or malignancy: cirrhosis is less likely.
• Patients with peritoneal dialysis: peritonitis diagnosed by dialysate white cell count over 100 per mm3 with at least 50% neutrophils.
• Regions: schistosomiasis, hepatitis B, aflatoxin exposure and tuberculosis are common causes, and tuberculous peritonitis is frequent in HIV patients.

Follow up tests. Serum albumin, liver function tests and INR, creatinine and sodium, blood cultures, hepatitis B and C serology, liver ultrasound with Doppler, abdominal CT, upper endoscopy for varices, echocardiography, urine protein, tuberculosis tests (Xpert, culture, adenosine deaminase), peritoneal biopsy or laparoscopy, CA 125 and CEA, and alpha fetoprotein.