ABSTRACT
Historically, the differential diagnosis of the origin of pleural effusions has relied upon the concepts of transudate and exudate (Table 2). Classically, transudative effusions occur without alterations in capillary permeability. Exudates are a result of increased capillary leak or diminished lymphatic clearance. Initially proposed in 1972, Light’s criteria classify an exudate as any fluid with one of the following characteristics:
Fluid/serum protein ratio > 0.5 Fluid/serum lactate dehydrogenase (LDH) ratio > 0.6 Absolute LDH > 2 / 3 upper limit of normal
A transudate meets none of these criteria. This classification scheme has a sensitivity and specificity of 98 and 82% respectively. A recent metanalysis confirms the use of the cutoff points for LDH and protein ratios used in Light’s criteria but modifies the fluid LDH cutoff to 45% of the upper limit of normal. The analysis suggests that other test combinations have similar diagnostic accuracy to Light’s criteria, specifically:
Fluid LDH > 0.45 upper limit of normal Fluid cholesterol > 45 mg/dL
Fluid protein > 2.9 g/dL Fluid LDH > 0.45 upper limit of normal Fluid cholesterol > 45 g/dL
No particular test combination is diagnostically superior. A strategy employing pleu ral fluid testing without serum testing has clear-cut cost savings and convenience benefits with equivalent diagnostic accuracy. A confounding factor encountered with the diagnostic separation of transudates and exudates is seen in diuretic-treated congestive heart failure, where the fluid protein can be elevated to the range of 3 to 4 g/dL. Fluid cholesterol has proven to be extremely helpful in classifying such effusions.
