In general, when an effusion occurs, the problem is differentiation among neoplastic, infectious, and fluid leakage etiologies. Effusions due to neoplasms or infection are frequently termed exudates and those due to hydrostatic leakage from vessels are called transudates. Several criteria have been proposed to separate transudates and exudates and to differentiate among the three major diagnostic categories. Most work has been done on pleural fluids. The significance of tests performed on pleural fluid may not be the same if the tests are performed on ascitic fluid.
Etiology. The two most common causes of pleural effusions are congestive heart failure and neoplasm. Infection (tuberculosis or pneumonia) is the third most frequent etiology. In some cases it is necessary to establish the diagnosis of chylous effusion. Chylous effusions usually have a triglyceride content of 110 mg/100 ml (1.2 mmol/L) or greater and are usually more than twice the serum triglyceride value. Centrifugation does not clear a supernate area, and it may be possible to demonstrate fat droplets with a fat stain such as Sudan III. Another problem that occasionally arises is to differentiate urine from effusion fluid. Urine almost always has a creatinine concentration twice that of serum or more, whereas effusion fluid usually has the same creatinine concentration as the patient’s serum or at least is not elevated as much as twice the serum level. Rarely, recurrent fluid in or draining from the nose or ear has to be differentiated between cerebrospinal fluid (CSF) leakage from the central nervous system (CNS) subarachnoid space versus a serum transudate or local mucosa secretion. The usual diagnostic test is injection of a radioisotope into the CSF and subsequent analysis of a specimen of the draining fluid. Some other tests may be the ratio between serum and CSF total protein (which is usually more than 100), serum albumin/CSF albumin ratio (which is usually over 200), and serum prealbumin/ CSF prealbumin ratio (which is usually over 14).
Specific gravity. Exudates typically have a specific gravity of 1.016 or more and transudates less than 1.015. One study found about 25% error in misclassification of either transudates or exudates.
Protein content. Pleural fluid total protein levels higher than 3 gm/100 ml (30 gm/L) are characteristic of exudates. Transudates have total protein content of less than 3 and usually less than 2 gm. Two studies found that 8% of exudates and 11%–15% of transudates would be misdiagnosed if 3 gm/100 ml were used as the dividing line. Most exudates that were misdiagnosed as transudates were neoplastic. A pleural fluid/serum protein ratio of 0.5 may be a slightly better dividing line; exudates usually have a ratio greater than 0.5. With this criterion, accuracy in identifying transudates improved, but 10% of the exudates, mostly of malignant origin, were incorrectly classified as transudates. Pulmonary infarct, rheumatoid-collagen diseases, acute pancreatitis, cirrhosis with high-protein ascites (12%–19% of cases), and other conditions may produce effusions with protein content compatible with exudates.
Several investigators report that the albumin gradient between serum and ascitic fluid differentiates between transudate or exudate nature of ascites better than total protein content. In one study, total protein ascitic values produced 64% overlap between etiologies of the exudate and transudate groups, whereas the serum albumin-ascitic fluid albumin gradient (SA-AFAG) produced 38% overlap. Another study produced only 7% overlap. The SA-AFAG consists of subtracting the ascitic albumin value from the serum albumin value. A SA-AFAG value of 1.1 gm/100 ml (11 gm/L) or more suggests a transudate, usually caused by portal hypertension due to cirrhosis. A SA-AFAG value less than 1.1 gm suggests an exudate but will not differentiate malignancy from infection or inflammation and occasionally may occur in nonmalignant, nonalcoholic cirrhosis. Another problem may arise when two conditions coexist such as liver metastases in a patient with cirrhotic ascites.
Patients with ascites due to cirrhosis develop bacterial infection of the ascitic fluid without known cause (“spontaneous bacterial peritonitis”) in about 15% of cases (range, 4%–20%). Spontaneous ascitic infection typically has an ascitic total protein less than 1.0 gm/100 ml (10 gm/L). Other types of ascitic fluid infection (“secondary peritonitis”) usually have an ascitic fluid total protein level greater than 1.0 gm/100 ml, ascitic fluid glucose less than 50mg/100 ml (2.78 mmol/L), and more than one organism obtained by culture. Gram stains of ascitic fluid are said to be positive in only 10% of spontaneous peritonitis, but more frequently in peritoneal fluid due to intestinal perforation.
Effusion lactic dehydrogenase. A pleural fluid to serum lactic dehydrogenase (LDH) ratio greater than 0.6 is reported to be typical of exudates. One study found that most transudates were correctly identified but that nearly 30% of exudates were misclassified.
Combinations of criteria. The more criteria that favor one category as opposed to the other, the more accurate the results become. One study found that the combination of pleural fluid/serum protein ratio and pleural fluid/serum LDH ratio correctly identified most transudates and exudates.
pH. An effusion fluid pH higher than 7.40 usually is associated with a transudate, whereas a pH of less than 7.40 is more likely to be an exudate caused by infection, inflammation, or tumor.
Glucose. A pleural fluid glucose level more than 10 mg/100 ml below lower limits of normal for serum, especially when the actual pleural fluid value is less than 20 mg/100 ml, is reported to be suggestive of neoplasm or infection. Possibly 15%–20% of malignant effusions have decreased glucose levels. Patient hypoglycemia, rheumatoid arthritis, and infection are other etiologies.
Cell count and differential. In ascites, a total WBC count of 250 mm 3 or more strongly suggests infection, especially when neutrophils exceed 50% of total WBCs (some use 500 WBCs as the cutoff point). In any body fluid, presence of many segmented granulocytes suggests infection (empyema); many mononuclear cells raise the question of lymphoid malignancy, carcinoma, or tuberculosis. However, several investigators state that sufficient exceptions occur to severely limit the usefulness of differential counts in the diagnosis of individual patients. One study reported that peripheral blood WBCs did not affect ascitic fluid WBC counts.
Culture. Culture is frequently performed for tuberculosis, fungus, and ordinary bacteria. Pleural fluid culture for tuberculosis is said to be positive only in approximately 25% of known cases of tuberculosis effusion. Some believe that tuberculosis culture should be limited to high-risk patients or patients who have a positive skin test result. Whereas tuberculosis is an important cause of idiopathic pleural effusion, although less common in the United States than in the past, fungus is an uncommon cause of pulmonary infection except in patient groups with compromised immunologic defenses. Studies have shown about 85% sensitivity of culture in ascitic infection using blood culture bottles inoculated at the time of paracentesis versus only 50% sensitivity when ascitic fluid is streaked on agar plates or inoculated onto broth media in the laboratory.
Cytology. About 30%–40% (literature range, 25%–52%) of all pleural effusions are associated with neoplasms. About 35%–40% are caused by lung carcinoma (most often adenocarcinoma), and about 20%–25% are due to breast carcinoma, with lymphoma or leukemia, ovary, or unknown primary in third place. Cytologic study is reported to detect tumor cells in about 50%–65% (literature range, 30%–98%) of patients with malignant pleural effusions. One problem that sometimes occurs is poor cytologic preparations due to blood in the pleural fluid. We have obtained better results by using cytologic spray fixative when the cytologic slides are prepared rather than fixing the slides by the usual technique of dipping them in alcohol.
Pleural effusion carcinoembryonic antigen (CEA) CEA is discussed in detail elsewhere. Pleural fluid CEA levels may be elevated in various malignant and some benign conditions. When a cutoff level approximately 4 times the upper reference limit (corresponding to 10 ng/ml with the Hansen technique, whose upper normal limit is 2.5 ng/ml) is used, most elevations due to nonmalignant cause are eliminated (< 5% false positive results; literature range, 1%–12%). About 35%–50% of malignancies are detected (25%–89%). Therefore, CEA by itself is less sensitive than cytology. Addition of CEA to cytology (using a CEA cutoff value sufficient to exclude benign disease) improves detection of malignancy about 10%–15% over cytology alone. Carcinoembryonic antigen assay can also be used for ascitic fluid, with similar results.
Tests for cancer-related ascites Among many tests proposed to detect malignancy causing ascites or accumulation of peritoneal fluid are the serum albumin-ascitic albumin gradient (SAAAG), ascitic fluid cholesterol, ascitic fluid fibronectin, cytology, CEA, flow cytometry (FCM), CA 125, and the monoclonal antibody immunohistochemical stains. In general, SAAAG less than 1.1 appears to be the best single overall relatively simple test, with sensitivity in detecting malignancy about 93% (range, 85%–100%) and accuracy of about 95% (range, 93%–97%). The main drawback is inability to detect those cases of ascites due to liver metastases or hepatocellular carcinoma without peritoneal implants (since the intrahepatic malignant cells are infrequently in direct contact with ascitic fluid) or differentiate these cases from ascities due to cirrhosis. Ascitic fluid cholesterol greater than 45 in two reports had 90%–100% sensitivity, but not enough studies are available, and patients with cardiac or pancreatic-origin ascities may in some cases have elevated ascitic cholesterol. Fibronectin had sensitivity of about 90% (range, 86%–93%) in three studies, but specimens usually would have to be sent to a reference laboratory. Serum CEA has been discussed earlier. Ascitic fluid CEA has a reported sensitivity of about 50% (range, 36%–80%). Cytology of ascitic fluid has sensitivity of about 60% (range, 40%–70%). Adding CEA assay to cytology increases cytologic sensitivity about 10%–20%. FCM estimates the amount of nucleic acid in cell nuclei; in general, an abnormal quantity of nucleic acid (aneuploidy) suggests malignancy. In one study, FCM aneuploidy increased the number of patients found to have malignancy by 39% over results of cytology alone. However, not all aneuploid cells are malignant, and not all malignant cells are aneuploid . Therefore, flow cytometry has been reported to produce about 30% (range, 0%–43%) false negative results and some false positive results. CA 125 assay in serum is discussed earlier in this chapter. It has much less often been applied to ascitic fluid. In a few reports, CA 125 has been reported to increase detection of ovarian carcinoma (and occasionally, uterine or fallopian tube carcinoma) over detection rates from cytology with or without CEA. Disadvantages of ascitic fluid CA 125 assay is frequent elevation of the antigen in ascities due to cirrhosis and to some extent with endometriosis. Monoclonal antibody stains against various tumor antigens have been applied to cell blocks or smears or by FCM in body cavity fluid specimens. The most useful antibodies in peritoneal fluid appear to be CA 125 and B72.3 for ovarian carcinoma, and EMA and CEA for adenocarcinoma in general. In one representative study, peritoneal washings from patients with stage I and II ovarian carcinoma were positive by cytology in 41% of patients and by immunohistology in 56%. In stage III and IV ovarian carcinoma, immunohistology also added an additional 14% positive patients to results from cytology.
Peritoneal lavage for traumatic injury. Although this subject does not involve cancer, it does fit with discussions on tests for effusions, and thus it is included here. The standard criteria leading to high expectation of intraabdominal bleeding are one or more of the following: aspiration of gross blood (the quantity required is not uniform, but at least 10 ml or 20 ml are most often mentioned), fluid with an RBC count greater than 100,000/mm 3, or a WBC count greater than 500/mm 3. Other criteria that have been proposed but that are not widely accepted are abdominal fluid bilirubin or creatinine values higher than serum values or elevated effusion amylase. In most series the standard criteria detect significant intraabdominal bleeding in about 90% of cases and falsely suggest significant bleeding in about 10%–15% of cases (some of these patients may have bleeding that retrospectively is not considered sufficient to warrant laparotomy). CT scanning has proved extremely useful in trauma patients, with a sensitivity equal to that of lavage and a false positive rate significantly less than that of lavage. In addition, CT can often demonstrate what organs are affected.
General considerations. Three anticoagulated tubes of effusion fluid should be sent to the laboratory, one tube containing ethylenediamine tetraacetic acid (EDTA) anticoagulant, one tube containing 0.05% sodium polyanetholesulfonate (SPS; Liquoid), and the third containing heparin. The EDTA tube is used for cell count and differential, the SPS tube for culture, and the heparinized tube for cytology. Without anticoagulant there may be sufficient protein in the specimen to induce spontaneous clotting, which can trap WBCs and bacteria and produce erroneous cell counts and falsely negative cultures. Some use of the heparinized tube both for culture and for cytology, but too much heparin may inhibit bacterial growth. Nonanticoagulated effusion fluid should also be sent to perform biochemical tests. As noted previously, when the effusion is ascites it is better to inoculate blood culture bottles when the ascitic fluid is obtained rather than to perform routine culture methods.
