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Clarity and color of the fluid should be noted. The clarity or opacity of the fluid depends largely on the presence and the amount of the neutrophil count or the presence of lipids. The amount of protein, bile, and bilirubin present will determine color. A “benign” fluid sample that has low protein and neutrophil count less than 250/mm3 is usually transparent and slightly yellow-tinged. The presence of bloody or serosanguinous fluid should be noted. A traumatic tap gives a bloody fluid that would clot easily in a nonanticoagulant-containing tube, while a blood-tinged nontraumatic ascitic fluid due to other reasons will not clot. Examples where bloody ascitic fluid may be expected are patients with hepatocellular carcinoma, trauma, and postsurgical and occasionally in peritoneal carcinomatosis. A milky or chylous fluid indicates a high triglyceride concentration and can be seen in advanced stage lymphomas. Bilirubin-stained ascitic fluid occurs in the setting of significant jaundice and appears tea-colored or dark brown. Bile-stained ascites appears greenish and is seen in patients with a bile leak and in patients with hemorrhagic or necrotic pancreatitis.
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Only a limited number of diagnostic tests need to be ordered to “profile” the nature of the ascites. A cell count, total protein, and albumin are generally sufficient in most cases and need not be repeated with each paracentesis. The most important test in the symptomatic patient is the cell count and only a few milliliters are needed for evaluation. It is usually agreed that in uncomplicated cirrhosis, the white blood cell (WBC) count will be less than 500/mm3 and the absolute polymorphonuclear neutrophil (PMN) count should be less than 250/mm3. A PMN count of greater than 250/mm3 is considered presumptive evidence of SBP pending culture results but it is important to note that 10% of cases of culture-proven SBP may occur with a PMN count of less than this value. A bloody or serosanguinous ascitic fluid is more difficult to interpret however; using a correction factor subtracting one PMN for each 250 red blood cells (RBCs) may be useful in some scenarios.
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Other basic diagnostic tests include Gram stain and culture (aerobic and anaerobic), albumin, total protein, and cytology. More specific tests for diagnosis are triglyceride level in chylous ascites, red blood cell count in trauma and malignancy, bilirubin concentration in bowel perforation, and amylase in pancreatitis (Table 104–2), but need not be ordered routinely.
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The serum-to-ascites albumin gradient (SAAG) helps to differentiate etiologies of ascites. The SAAG can be calculated by subtracting the ascitic fluid albumin value from the serum albumin, both measured on the same day. SAAG above 1.1 g/dL suggests the presence of portal hypertension. Etiologies of ascites that are associated with portal hypertension include cirrhosis, congestive heart failure, portal vein thrombosis, and Budd-Chiari syndrome. SAAG less than 1.1 g/dL indicates absence of portal hypertension. Examples of causes of ascites without portal hypertension are nephrotic syndrome, peritoneal tuberculosis, and carcinomatosis (Table 104–3). In addition to the ascitic fluid albumin, a total ascites protein may be used to assess the risk of SBP with an increased risk associated with levels less than 1 g/dL. It is felt that this level predisposes to SBP because it correlates with complement levels and opsonic activity.
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