PLEURAL FLUID ANALYSIS

• Pleural fluid analysis is often indicated to diagnose causes of pleural effusions.
• Pleural fluid is obtained via thoracentesis, a procedure performed (often at bedside with sterile techniques) by inserting a needle into the pleural cavity to drain out as much fluid as possible to both relieve discomfort (therapeutic benefit) and obtain pleural fluid for analysis.
• The first step in the analysis is to determine whether the fluid is transudative or exudative using the Light’s criteria.
• Additional information can be obtained via pleural fluid gross appearance, cell count with differential, glucose level, pH level, cytology, tumor markers, microbiology, and other tests.

• Pleural effusions can be either transudative or exudative, depending on the underlying pathophysiological mechanism:
   Transudate: fluid that is forced out of the capillary into the pleural space due to either an increase in intravascular hydrostatic pressure (e.g. in heart failure, cirrhosis) or a decrease in intravascular osmotic pressure (e.g. in nephrotic syndrome). Fluid can also enter the pleural space from the peritoneal, retoperitoneal, cerebrospinal space, or infusion via central venous catheter
   Exudate: fluid with high protein content leaked into the pleural space across the highly permeable capillary membrane as a result of inflammation (e.g. malignancy, infection, autoimmune disorders, etc) but is not adequately drained by the lymphatic system due to various causes such as blockage or injury
• Light's criteria is routinely used to differentiate between these 2 types of pleural fluid. Protein and lactate dehydrogenase (LDH) levels in pleural fluid are compared with the corresponding serum levels (proteins and LDHs are used as surrogates for enzymatic activities of malignant and inflammatory cells, which are the pathological factors of exudative pleural effusions).
• Per Light's criteria, to be classified as “exudate”, 1 of the 3 criteria must be met:
   1) pleural protein/serum protein level is greater than 0.5
   2) pleural LDH/serum LDH level is greater than 0.6
   3) pleural LDH is greater than 2/3 of upper limit of serum LDH

• Additional tests can be done to further characterize exudative fluid and to determine etiologies including cell count with differential, glucose, pH, cytology, tumor markers, microbiology, amylase, NT-proBNP, triglyceride, cholesterol, among others.