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The serous body cavities are mesothelial lined potential spaces surrounding the lungs, heart and abdomen. Normally, they contain a small amount of fluid that is an ultrafiltrate of plasma. When production and resorption of this ultrafiltrate are not balanced, fluid may accumulate, resulting in an effusion. Effusions may be classified as transudates or exudates. Transudates are usually bilateral and arise from either increased capillary hydrostatic pressure or decreased oncotic pressure secondary to congestive heart failure, fluid overload, cirrhosis or hypoalbuminemia. Exudates are usually unilateral and result from increased capillary permeability or decreased lymphatic resorption associated with infection, connective tissue disease, pancreatitis or cancer. Several laboratory tests are helpful in distinguishing transudates from exudates including pH, total protein, lactate dehydrogenase (LD), amylase, glucose, white cell count and differential. Only one of these values has to fall into the exudate range for the effusion to be classified as an exudate. Large chemistry panels should not be ordered on body fluids. Light's criteria have been widely used to classify pleural and pericardial fluid as a transudate or an exudates. Light's Criteria
Exudates typically have higher protein concentration and LD activity and lower pH and glucose values than transudates. The protein concentration of an exudate usually exceeds 3 g/dL. Exudate LD activity is greater than 0.67 times the upper limit of normal for serum. Other tests may also be helpful in evaluating pleural effusions. Pleural fluid pH is useful to evaluate the prognosis of effusions associated with pneumonia. Normal pleural fluid pH is 7.6. A pleural fluid pH > 7.3 suggests that resolution is possible with medical therapy alone. A pH < 7.2 suggests that a more complicated effusion or empyema requiring surgical drainage has probably formed. Pleural fluid pH should be measured with a blood gas analyzer and not with litmus paper or a pH meter, because both of the latter methods result in falsely elevated values. A pleural fluid glucose < 60 mg/dL or a pleural fluid: serum glucose ratio < 0.5 may be seen in effusions caused by cancer, tuberculosis, empyema and rheumatoid arthritis. Triglyceride levels >110 mg/dL and the presence of chylomicrons indicate a chylous effusion. The criteria for classifying transudates and exudates in pleural and pericardial fluids are often misleading when applied to peritoneal fluid (ascites). Transudative processes may produce a peritoneal fluid protein level in the exudate range. Calculation of a serum to ascites albumin gradient (SAAG) is a more physiologically appropriate test. It is calculated as the serum albumin concentration minus the peritoneal fluid albumin. A high gradient ( > 1.1 g/dL) indicates ascites related to portal hypertension, usually due to cirrhosis. A low gradient < 1.1 g/dL is usually associated with peritoneal seeding by cancer, tuberculosis and nephrotic syndrome. Ascites fluid with an amylase level more than 3 times the serum value is usually caused by pancreatitis, pancreatic pseudocyst or trauma. Elevated bilirubin may indicate biliary tract injury. Elevated cholesterol in ascites fluid has been associated with malignancy. Elevated alkaline phosphatase has been associated with bowel injury. Total leukocyte and RBC counts are of limited value in body fluid analysis except when diagnostic peritoneal lavage is performed. Normally, less than 10,000 RBCs/uL are present in pleural, pericardial and peritoneal fluids. Pleural and pericardial fluids usually have less than 1000 WBC/uL while peritoneal fluid has less than 500 WBC/uL. RBC counts > 100,000/uL in pleural fluid are suggestive of malignancy, trauma, or pulmonary infarct. Lavage fluids usually have less than 50,000 to 100,000 RBCs/uL and less than 200 WBC/uL. Higher RBC and WBC cell counts in lavage fluids are consistent with hollow organ perforation.
According to literature related specifically to nephrology and dialysis, normal peritoneal dialysis fluid should contain less than 50 WBC/uL and have fewer than 15% neutrophils. A fluid should be considered suspicious for bacterial infection, when the total WBC count is >100/uL and the percentage of neutrophils exceeds 50%, Common nonbacterial causes of neutrophilia in dialysis fluids include infectious diarrhea, active colitis, menstruation or ovulation, and pelvic inflammatory disease. Gram stain should also be performed, but the sensitivity for bacterial peritonitis is only 50%. However, a positive gram stain is predictive of culture results in 85% of cases. Differential count is performed to determine the predominant cell type present in the fluid, which can suggest certain diseases. Reference ranges have not been established. Increased numbers of neutrophils are seen with exudates caused by bacterial infection, infarction, cancer or pancreatitis. Increased numbers of lymphocytes are associated with viral infections, tuberculosis, lymphoproliferative disorders, congestive heart failure, and cirrhosis. Eosinophils are increased in infections, neoplasms, chronic renal failure, pneumothorax, pulmonary infarction and parasitic infestations. Plasma cells are present in rheumatoid arthritis, cancer, tuberculosis, and multiple myeloma. |
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