and Ian A. D. Bouchier2
(1)
Bishop Auckland, UK
(2)
Edinburgh, Midlothian, UK
The aetiology of ascites may be obvious from the history and physical examination. However, it is generally necessary to examine the fluid microscopically, chemically and bacteriologically. Even when the cause is clinically apparent, for example hepatic cirrhosis and portal hypertension, it may not be possible to exclude either superimposed infection or hepatocellular cancer.
16.1 Paracentesis
Diagnostic paracentesis is a simple technique and can easily be undertaken in all patients presenting for the first time with ascites unless there is a specific contraindication.
16.1.1 Method
The patient should be asked to empty the bladder if not catheterized. The usual site for aspiration is in the right or left lower quadrant midway between the umbilicus and the anterior superior iliac spine. A 21 gauge needle may be used to inject the local anaesthetic and a similar size needle can then be inserted through the peritoneum for the paracentesis. Other suitable needles are those used for lumbar and cisternal puncture. When there is a very tense ascites it is often possible to insert a fine needle without using local anaesthetic. After a sufficient volume of fluid has been withdrawn for examination the needle is removed and a gauze dressing is applied to the wound.
16.1.2 Complications
These are rare. Occasionally an abdominal wall vein is penetrated. The procedure may be followed by a leak of fluid from the injection site when the ascites is very tense. A skin suture inserted after aspiration may prevent this, but not infrequently the leak only stops when the ascites has been relieved.
16.1.3 Interpretation
16.1.3.1 Appearance
Ascitic fluid which is a transudate is clear and straw-coloured. An exudate may also be clear but the fluid is usually cloudy and opalescent because of a high cell content. Trauma, malignant disease or tuberculous disease of the peritoneum may cause the fluid to be bloodstained. The fluid has a high mucoid content when pseudomucinous tumours have invaded the peritoneum.
16.1.3.2 Microscopic Examination
Five millilitres of the ascitic fluid are added to a tube containing anti-coagulant, centrifuged for 10 min and a smear made of the deposit. A rough estimate is made of the number of cells and a differential count is undertaken. The presence of many polymorphonuclear leucocytes suggests non-tuberculous infection while a high lymphocyte count suggests tuberculosis or lymphoma. The unstained smear may be examined for microfilaria and trypanosomes, or it can be fixed and stained with either Leishman or Giemsa stain.
The spun deposit may be stained and examined for malignant cells by a trained cytopathologist. An accuracy of about 86% correct positive diagnosis is achieved. There is much difficulty in identifying cells when there has been ascites of long duration such as with cirrhosis of the liver. Exfoliated mesothelial cells are a particular cause of confusion and can be mistaken for malignant cells.
A counting chamber can be used for cell counts, but caution must be exercised in the interpretation of the result when there is contamination with red blood cells. In transudates, for example in alcoholic liver disease, the mean cell count is 280 mm3. In exudates the count is usually >500/mm3. Exudates associated with carcinoma have an average cell count of 690/mm3 (with mixed cellularity); tuberculous exudates characteristically contain many lymphocytes (92%). Two conditions with very high counts, averaging 7000/mm3, are lymphomas where nearly 70% of cells are lymphocytes, and spontaneous bacterial peritonitis in which the cells are almost entirely polymorphs. If the count is <250/mm3 the ascites is sterile, and peritonitis can only be confidently diagnosed when counts exceed 1000/mm3.
16.1.3.3 Chemical Analysis
A protein content <25 g/l suggests that the fluid is a transudate. This is usually the case in heart failure, cirrhosis of the liver, nephrosis and other conditions associated with severe hypoproteinaemia.
A protein concentration >25 g/l suggests the presence of an exudate. This is found in acute peritoneal infections, tuberculous peritonitis and metastatic malignant disease involving the peritoneum. Fluid with a high protein content is occasionally encountered in cirrhosis in the absence of infection or malignant disease. The ascitic fluid in patients with myxoedema and endomyocardial fibrosis may contain a high level of protein.
The amylase concentration of the ascitic fluid may be increased in patients with acute pancreatitis and pancreatic pseudocyst. Occasionally a perforated peptic ulcer will be associated with amylase-rich ascitic fluid.
16.1.3.4 Bacteriological Examination
At least 10–20 ml of the ascitic fluid is sent for culture. When tuberculosis is suspected a large volume of the fluid is sent to the laboratory in a bottle containing sodium citrate to prevent the fluid from clotting. TB may be sought by smear, culture or guinea-pig innoculation, but the diagnosis of tuberculous peritonitis is established by bacteriological methods in only 50% of patients.
16.1.4 Diagnostic Paracentesis in the Acute Abdomen
The technique is a modification of that used when there is ascites. A 21-gauge needle is inserted under local anaesthesia into the peritoneal cavity at four sites: the right and left upper and lower quadrants midway between the umbilicus and the anterior superior iliac spines below and the ninth costal cartilage above. Gentle suction is applied using a 2 or 5 ml syringe while the needle is moved about within the peritoneal cavity. The appearance and volume of the aspirate is noted and the material sent for biochemical and bacteriological analysis.
Normally <0.5 ml clear fluid can be aspirated. A volume exceeding 0.5 ml of fluid which is obviously abnormal suggests intra-abdominal disease. A negative paracentesis has no diagnostic significance. The technique is of value in the diagnosis of acute intraperitoneal haemorrhage, as in acute pancreatitis when pure blood is aspirated that fails to clot. Paracentesis is especially helpful in the management of patients with non-penetrating abdominal injury. Alkaline bile-stained fluid, often containing food debris, is characteristic of a perforated peptic ulcer. The technique is not useful in the diagnosis of localized inflammatory disease.
The procedure is safe although the intestine may be accidentally penetrated when there are many adhesions or if there is a malignant peritonitis. This is usually readily appreciated from the appearance and microscopy of the aspirate.
16.1.5 Chylous Ascites
The aspiration of an opalescent, cloudy fluid suggests the possibility of a chylous ascites, which foIlows a leak of lymph into the peritoneal cavity. Chylous fluid contains absorbed fat (>5 mmoI/l) in the form of particulate chylomicrons which float on standing. This must be distinguished from pseudochylous ascitic fluid which is opalescent because it contains fat and granular material derived from degenerated cells (which tend to sediment). Chronic chylous ascites is associated with malignancy in 80% of cases. A wide variety of causes may underlie subacute chylous ascites.
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