BACKGROUND: Classification of pleural effusions into transudates and exudates is based on pleural fluid absolute lactic dehydrogenase value (FLDH), fluid to serum ratio of LDH (LDHR) and fluid to serum ratio of total protein (TPR) used in a parallel combination strategy. Combining multiple tests in a parallel strategy to improve diagnostic accuracy is useful only if the pair-wise correlation of the individual tests is less than 0.75. So far, this concept has not been tested in patients with pleural effusions. MATERIAL/METHODS: Biochemical data from our 200-patient series with a known cause of pleural effusion were included in this study. Correlation between the three possible combinations of tests was determined. RESULTS: There were 116 males and 84 females. The mean age was 62+/-1.1 years (mean+/-SEM). Of the 200 effusions, 156 were exudates and 44 were transudates. There was a significant correlation between FLDH and LDHR (r=0.93, p

Keywords: Diagnosis, Differential, Exudates and Transudates, L-Lactate Dehydrogenase - blood, Pleural Effusion - blood, Pleural Effusion - classification