Interpreting sample results in ascites
- There is no ‘standardized’ fluid cell count
- The generally accepted ‘cut-off’ for the upper-limit of normal is <250 neutrophils/mm3. Neutrophils usually constitute 70% of the cell count.
- In spontaneous bacterial peritonitis, neutrophils are the predominant line. In TB ascites, lymphocytes predominate.
- Bloody ascites is usually the result of the procedure itself, i.e. a traumatic tap. (Remember the anatomical position of the superficial epigastric arteries).
Exudates, Transudates and Ratios
- Patients with an ascitic total protein gradient of <15g/L have an increased risk of developing spontaneous bacterial peritonitis and may benefit from antibiotic prophylaxis.
- Traditionally the serum: ascites LDH and protein ratios help differentiate exudates from transudates. However, the literature shows that these calculations are actually not helpful.
- The Serum Ascites Albumin Gradient (SAAG), a concept involving oncotic-hydrostatic balance, has become the preferred measurement to characterize ascites.
Serum Ascites Albumin Gradient (SAAG) = [Albumin]serum – [Albumin]ascites
|SAAG > 1.1 mg/dl (Portal Hypertension)||SAAG < 1.1 mg/d|
- You would expect the ascitic amylase to be about half the serum amylase
- If the ascites is secondary to pancreatitis or perforated viscus, the ascitic amylase can be as great as five-fold higher than serum amylase
- In uncomplicated ascites this should be similar to serum levels.
- In advanced (but often not in early) Spontaneous Bacterial Peritonitis, glucose levels in ascites can drop to as low as 0 mg/dl as a result of bacterial consumption
Cultures and Gram Stain
- Cultures should be obtained by inoculating blood culture bottles at the bedside. This has been shown to improve sensitivity to at least 80%, compared with 50% for ‘conventional’ culture methods.
- Gram stains are relatively useless on ascites fluid – about as useful as asking for a Gram stain on blood cultures to look for bacteremia. The concentration of organisms just won’t be high enough to see something on Gram stain.
- Truly only helpful in diagnosing peritoneal carcinomatosis with reported sensitivities of ≤100%.
- Does not detect most other intra-abdominal cancers, mainly because most do not frequently metastasize to the peritoneum.
- Think of cytology as helping you only if you’re suspicious of a cancer that has spread to the peritoneum. And remember that a negative cytology does not rule out cancers such as hepatocellular carcinoma or liver metastases, which commonly cause ‘malignant ascites.’