Content area
Full Text
Pleural effusions develop in thoracic or systemic diseases and, based on their underlying pathophysiology, they are classified into transudates or exudates. 1 2 They are termed exudates if the pleural fluid to serum ratio of total protein (TPR) is >0.5, the pleural fluid absolute lactic dehydrogenase (FLDH) level is >200IU/l, or the pleural fluid to serum ratio of LDH (LDHR) is >0.6, and transudates if the TPR is <0.5, FLDH level is <200IU/l, and the LDHR is <0.6. 3
The accuracy of these tests has been analysed using conventional histograms and was reported to approach 100%. 4 However, these analyses were done using a single sensitivity and specificity derived from a scatter plot. Calculation of the accuracy based on a single sensitivity/specificity pair provides only a brief glimpse of the performance of a test and does not reveal its true diagnostic value. 5 Furthermore, in the diagnostic separation of pleural effusions into transudates and exudates, the performance of these tests was not compared individually or in combination. We have therefore analysed the global performance of these tests using receiver operator characteristics (ROC) analysis in a heterogeneous group of patients with a known cause for their pleural effusion. The TPR value was chosen instead of the absolute value of fluid total protein to differentiate the pleural effusions, based on the observation that the total protein concentration in the pleural fluid is influenced by changes in the protein concentration in the serum. 3 However, the scientific basis for using LDHR has not been explored by previous investigators. 3
Methods
Patients referred to the respiratory unit at Rotherham General Hospital, UK for a prospective investigation of the dynamics of pleural effusion formation and removal from January 1989 to June 1991 were included in the study. 6 The cause of the pleural effusion was determined using established clinical criteria. 3 All patients were followed for at least 3 months or until a final cause of the pleural effusion was established. Blood and pleural effusion samples collected and stored were later analysed for glucose, albumin, total protein, and LDH. LDH was measured with a Boehringer Mannheim kit according to established methods and the results were expressed in IU/l. 7 The upper limit for the normal serum LDH level in our...