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. 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 light's 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. light's 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 light's 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. therapeutic aspiration may be sufficient; larger effusions may require insertion of an intercostal drain (either pigtail or surgical). when managing these chest tubes, it is important to make sure the chest tubes do not become occluded or clogged. a clogged chest tube in the setting of continued production of fluid will result in residual fluid left behind when the chest tube is removed. this fluid can lead to complications such as hypoxia due to lung collapse from the fluid, or fibrothorax if scarring occurs. repeated effusions may require chemical (talc, bleomycin, tetracycline/doxycycline), or surgical pleurodesis, in which the two pleural surfaces are scarred to each other so that no fluid can accumulate between them. this is a surgical procedure that involves inserting a chest tube, then either mechanically abrading the pleura or inserting the chemicals to induce a scar. this requires the chest tube to stay in until the fluid drainage stops. this can take days to weeks and can require prolonged hospitalizations. if the chest tube becomes clogged, fluid will be left behind and the pleurodesis will fail. pleurodesis fails in as many as 30% of cases. an alternative is to place a pleurx pleural catheter or aspira drainage catheter. this is a 15fr chest tube with a one-way valve. each day the patient or caregivers connect it to a simple vacuum tube and remove from 600 to 1000 ml of fluid, and can be repeated daily. when not in use, the tube is capped. this allows patients to be outside the hospital. for patients with malignant pleural effusions, it allows them to continue chemotherapy if indicated. generally, the tube is in for about 30 days, and then it is removed when space undergoes spontaneous pleurodesis. tubercular pleural effusion is one of the common extrapulmonary forms of tuberculosis. treatment consists of antituberculosis treatment (att). the currently recommended att regime is two months of isoniazid, rifampicin, ethambutol and pyrazinamide followed by four months of isoniazid, rifampicin and ethambutol.