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<0.00). However, the correlation between FLDH and TPR (r=0.27) and TPR and LDHR (r=0.22) was not significant.
Conclusions: The operative mechanism for LDHR and FLDH used in the classification of transudate and exudates appears to be similar, and therefore unsuitable for a parallel combination strategy in the diagnostic separation of pleural effusion. FLDH and TPR have a dissimilar operating mechanism, and can therefore be combined in this process. Therefore, the diagnostic separation of pleural effusion can be done cost effectively by utilizing FLDH and TPR alone, as the cost for estimating serum LDH is eliminated in this approach.