Postural changes in pleural fluid constituents.

Measurement of pleural fluid constituents (pH, PCO2, protein, lactic dehydrogenase [LDH], glucose, red blood cells [RBC], and white blood cells [WBC]) are of value in the diagnosis of pleural effusions and in the separation of exudates from transudates. The position of the patient (sitting or lying) prior to thoracentesis may result in differences in the measurement of these constituents. We found significant postural changes (p < 0.05) in the measurement of pH, protein, LDH, and RBCs in all exudative effusions (n = 12) in the upright sitting position compared with the supine position. No postural changes were noted in the transudative effusions (n = 6). The location of basal pleural tumors and the molecular weight of certain constituents may explain this postural sedimentary effect. Repeated thoracentesis after 30 min in the sitting position may result in exudative values when borderline transudates are found in a patient previously in a supine position.