Study objective: To determine the diagnostic value of pleural fluid la
ctate dehydrogenase (LDH) isoenzyme analysis in the differential diagn
osis of pleural fluid. Patients and methods: Eighty-seven consecutive
patients with pleural effusion caused by congestive heart failure (33)
, infection (33), and malignancy (21) comprised a derivation set of pa
tients. Pleural fluid LDH activity and isoenzyme pattern were establis
hed in all patients and analyzed by the classification and regression
trees (CART) method. An additional group of 20 consecutive patients co
mprised a validation set that was used for cross-validation of CART-de
rived decision tree. Results: A decision tree, with a positive predict
ive value of 83%, was constructed and validated by data from a validat
ion set of patients. Conclusions: Pleural fluid LDH isoenzyme pattern
may be helpful for the differential diagnosis of the most common cause
s of pleural effusions: congestive heart failure, infections, and mali
gnancy.