Recently, investigators from the Mayo Clinic published their biochemical analysis of the pleural fluid obtained from 74 adults with a diagnosis of chylothorax Mayo Clin Proc. Feb ;84 2 Gross appearance of the fluid was not a sensitive diagnostic criterion in identifying chylothorax. A nonmilky appearance should not be used as a criterion to rule out a chylous effusion. The traditional biochemical criterion for chylothorax is a pleural fluid triglyceride level greater than 1.
The Mayo study validated this criterion. 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. 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. Many patients do not manifest symptoms such as abdominal pain, fever or encephalopathy at the time of presentation. Therefore, it is recommended that all patients with cirrhosis and ascites undergo a paracentesis at the time of admission to assess for SBP.
An elevated PMN count alone is sufficient to establish the diagnosis as ascitic and blood cultures are often negative. Total leukocyte and RBC counts are of limited value in body fluid analysis except when diagnostic peritoneal lavage is performed.
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.
Transudate or Exudate Blood Sciences Test Protocol Sample Samples should ideally be collected into heparin tubes to prevent clot formation. Blood stained samples are not suitable for analysis of total protein and LDH.
All samples should be centrifuged prior to analysis. Interpretation The serous body cavities are mesothelial lined potential spaces surrounding the lungs, heart and abdomen. Pleural Effusions Other tests may also be helpful in evaluating pleural effusions. 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, autoimmune 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. Exudates typically have higher protein concentration and LD activity and lower pH and glucose values than transudates.
Exudate LD activity is greater than 0. Pleural fluid provides surface tension between the visceral and parietal pleura and assures close apposition and mechanical coupling between lung and chest wall.
It also serves as a lubricant preventing friction between pleural surfaces. Pleural fluid is continuously renewed. An ultrafiltrate of plasma moves from capillaries in the parietal pleura into the pleural space. Excess fluid is normally drained by lympatics in the parietal pleural. In a healthy person, the volume of pleural fluid around both lungs is approximately 0. A healthy adult weighing 75 kg would have approximately 10 mL of pleural fluid.
Between 10 and 20 mL of fluid is produced per day. The most common causes of pleural effusion are congestive heart failure, pneumonia, cancer, cirrhosis with ascites, and coronary artery bypass graft. Effusions can be classified into transudates and exudates based on the results of laboratory tests. The first 3 criteria are known as Lights criteria. Misclassification is most commonly seen in patients on long term diuretic therapy for congestive heart failure because dieresis concentrates pleural fluid protein and LDH.
Pleural fluid is more alkaline than blood because of its higher bicarbonate concentration. White blood cells mostly consist of macrophages and lymphocytes. Normal pleural fluid pH ranges between 7. Transudates usually have a pH between 7. Most bacterial infections result in a pH in the exudative range.
One exception is infection with Proteus which produces an alkaline pH between 7. Pleural fluid pH is useful to evaluate the prognosis of effusions associated with pneumonia. Pneumonia due to Proteus species is the exception to this rule because these bacteria produce urease that converts urea to ammonia, making the fluid alkaline pH 7.
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.
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