The traditional definitions of transudate as a pleural effusion due to systemic factors and an exudate as a pleural effusion due to local factors have been used since 1940 or earlier (Light et al., 1972). Previous to Lights landmark study, which was based on work by Chandrasekhar, investigators unsuccessfully attempted to use other criteria, such as specific gravity, pH, and protein content of the fluid, to differentiate between transudates and exudates. Lights criteria are highly statistically sensitive for exudates (although not very statistically specific). More recent studies have examined other characteristics of pleural fluid that may help to determine whether the process producing the effusion is local (exudate) or systemic (transudate). The table above illustrates some of the results of these more recent studies. However, it should be borne in mind that Lights criteria are still the most widely used criteria.The Rational Clinical Examination Series review found that bilateral effusions, symmetric and asymmetric, are the most common distribution in heart failure (60% of effusions in heart failure will be bilateral). When there is asymmetry in heart failure-associated pleural effusions (either unilateral or one side larger than the other), the right side is usually more involved than the left. The instruments pictured are accurately shaped, however, most hospitals now use safer disposable trocars. Because these are single use, they are always sharp and have a much smaller risk of cross patient contamination. Treatment
Treatment depends on the underlying cause of the pleural effusion.
1972), a pleural effusion is likely exudative if at least one of the following exists:
The ratio of pleural fluid protein to serum protein is greater than 0.5
The ratio of pleural fluid LDH and serum LDH is greater than 0.6
Pleural fluid LDH is greater than 0.6  or 2⁄3 times the normal upper limit for serum. Different laboratories have different values for the upper limit of serum LDH, but examples include 200 and 300 IU/l.The sensitivity and specificity of Lights criteria for detection of exudates have been measured in many studies and are usually reported to be around 98% and 80%, respectively. This means that although Lights criteria are relatively accurate, twenty percent of patients that are identified by Lights criteria as having exudative pleural effusions actually have transudative pleural effusions. Therefore, if a patient identified by Lights criteria as having an exudative pleural effusion appears clinically to have a condition that usually produces transudative effusions, additional testing is needed. In such cases, albumin levels in blood and pleural fluid are measured. If the difference between the albumin level in the blood and the pleural fluid is greater than 1.2 g/dL (12 g/L), this suggests that the patient has a transudative pleural effusion. However, pleural fluid testing is not perfect, and the final decision about whether a fluid is a transudate or an exudate is based not on chemical analysis of the fluid, but an accurate diagnosis of the disease that produces the fluid.