There are no Reference Intervals for the analytes measured in ASF, PDF and PLF described in this document, as such fluid is, by definition, pathological. The clinical uses for such testing vary greatly with fluid being tested and analyte sought. Clinical utility for each specific analyte is detailed in the individual method technical documents, derived from peer reviewed sources ((see in-house method documents, Reference No. 5 – 18).
Pleural fluid: Pleural fluid is clinically validated for protein and lactate dehydrogenase, used to determine an exudate vs a transudate pleural effusion via Light’s Criteria. Rarely Albumin can be used as an adjunct, if misclassification is suspected. Cholesterol as a supportive indicator of transudate vs exudate effusion, and in combination with triglyceride is used to differentiate between a chylous and a pseudochylous effusion. Pleural fluid pH is a sensitive marker for sepsis at a clinical cut-point of 7.20, pleural fluid glucose adds little if pleural fluid pH is available and < 7.2, but can be useful in the absence of a reliable pH measure. A low glucose (<4.3 mmol/L) in pleural fluid has high diagnostic accuracy for parapneumonic effusions, and (<1.6 mmol/L) in pleural fluid suggests either rheumatoid arthritis or empyema.
Ca19-9 and CEA is measured in pleural and ascitic fluid to support a diagnosis of malignancy as a cause for fluid in these cavities. Specificity is generally better than sensitivity, and the sensitivity of CEA is better than that for CA 19-9.
In the peritoneal cavity, ascites is most frequently analysed for albumin, to calculate a Serum-Ascites Albumin Gradient ([Serum Albumin]–[Ascitic Albumin]). Separating ascites into high-gradient and low-gradient fluids aids in narrowing the differential diagnosis. Similar to pleural fluid, low peritoneal glucose levels are measured when tubercular ascites or spontaneous bacterial peritonitis is suspected (clinical cut-point: 2.8 mmol/L). A ratio of fluid: serum glucose of < 1.0 can indicated spontaneous bacterial peritonitis, and a ratio of < 0.7 had, in one study, a sensitivity and specificity of 100% for the diagnosis of tubercular ascites.
Fluid Amylase can be a useful diagnostic marker of fluid of pancreatic origin. If the fluid is of pancreatic origin, amylase should be present in very high concentrations in body fluids, relative to serum enzymes, even in the case of acute pancreatitis. Amylase fluid was a good marker of complications from pancreatic leakage post operation.
Lipase is also measured in ascites as a marker for pancreatitis. Levels are very high in peritoneal fluid during pancreatitis compared to serum, even in cases of acute pancreatitis. It can be assayed for if pancreatic leak is suspected post operatively.
Uric acid can be used (with lower sensitivity and specificity) in place of light’s criteria to differentiate transudates from exudates.
Creatinine and urea are measured in pleural and peritoneal fluid, and compared to the serum analyte. A relatively high concentration relative to serum is considered a sensitive and specific test for the leakage of urine from an acquired defect in the urinary tract. Creatinine and urea are also measured in peritoneal dialysis to inform the clinicians to the transport characteristic category, and adequacy of dialysis respectively. Peritoneal sodium and potassium can also be used in this fluid, to assess adequacy of removal of these analytes. | 
