Table 3 lists the percentages of misclassified pleural effusions in every diagnostic group for every parameter studied. It shows that the criteria of Light et al, with their high sensitivity for exudates of any diagnostic origin, afforded the smallest percentage (5 percent) of misclassified pleural efiusions.
Looking for new cutoff points that could improve the discriminating capacity of the criteria of Light et al, we found that using the following as new cut-off points — 0.6 for the pleural fluid to serum protein ratio, 280 IU/L for LDH pleural fluid concentration, and 0.9 for pleural fluid to serum LDH ratio—the accuracy of the test remained practically unmodified (95 percent to 94 percent) while the specificity rose from 77 percent to 93 percent (Table 4).
In Table 5, patients with transudative efiusions due to CHF are grouped according to previous therapeutic use of diuretics. One patient was excluded because we could not ascertain previous use of diuretics. It shows that the use of diuretics does not significantly change the mean values of the different parameters used. Among the 21 patients with diuretics, the criteria of Light et al correctly classified 14 patients and misclassified 7 patients. The proportion of patients receiving long-term diuretics (more than a week) was not significantly different between groups.
The use of several chemical tests to separate transudates from exudates is recommended as a useful first step in determining the cause of a pleural effusion. In their initial study in 1972, Light et al used pleural and serum levels of proteins and LDH to establish criteria for segregating transudates from exudates with a sensitivity and specificity both near 100 percent. This high diagnostic accuracy together with a relatively low cost made the criteria of Light et al the “gold standard” for initial categorization of pleural effusions. Recently, however, several prospective studies were unable to reproduce the excellent results obtained by Light et al. In most of them, using the criteria of Light et al, the sensitivity for exudates remained high, greater than 95 percent, but the specificity did not surpass 78 percent. Because the pleural concentration of cholesterol, albumin, and bilirubin had shown a high specificity in all these studies, these parameters were proposed as alternatives to the classic criteria.
To the best of our knowledge, the present study included more patients than any previous series. The percentage of patients excluded due to multiple or unknown diagnosis (15 percent) falls between that of Light et al (18 percent) and Hamm et al or Valdes et al (11 percent). The absence of forced inclusion of those patients thought to have transudative pleural effusions explains a smaller number of patients with transudates than in previous series (Table 6).
Table 3—Number of Misclassified Pleural Effusions in Each Group Diagnostic for Every Parameter Studied
|PF/S-P (%)||PF-LDH (%)||PF/S-LDH (%)||Criteria of Light et al (%)||PF-Chol (%)||PF/S-Chol (%)|
|Transudates||4/44 (9)||0/44 (0)||8/44 (18)||10/44 (23)||4/44 (9)||12/42 (29)|
|Neoplastic-||17/132 (13)||48/132 (36)||9/130 (7)||2/130 (2)||25/132 (19)||18/132 (14)|
|Parapneumonic||9/49 (18)||22/48 (46)||7/48 (15)||2/48 (4)||15/49 (37)||8/48 (17)|
|Tuberculosis||1/43 (2)||5/43 (12)||0/43 (0)||0/43 (0)||3/43 (7)||0/40 (0)|
|Others||3/28 (11)||12/29 (41)||2/28 (7)||0/28 (0)||4/29 (14)||1/29 (3)|
|Total||34/296 (11)||87/296 (29)||26/293 (9)||14/293 (5)||51/297 (17)||39/291 (13)|
Table 4—Sensitivity and Accuracy of Modified Criteria of Light et al: Comparison With Classic Criteria of Light et al
|Criteria of Light et als||98||77||95|
|Modified criteriaf of||94||93||94|
|Light et al|
Table 5—Use of Diuretics in Transudative Pleural Effusions Secondary to CHF: Its Effects on Fluid Chemistry
|With (n = 21), mean ± SD||Without (n = 9), mean ± SD|
|PF-protein (g/dl)||2.36 ±0.86||2.32 ±0.65||NS|
|PF-chol, mg/dl||37.23 ±14.7||44.44 ±19.2||NS|
Table 6—Number of Pleural Effusions Studied, Percentage of Excluded and Percentage of Transudates: Comparison With Previous Series
|Source||Total||Excluded, No. (%)||Included||Transudates, No. (%)|
|Light et al||183||33 (18)||150||47 (31)|
|Hamm et al||70||8 (11)||62||31 (.50)|
|Valdes et al*||283||30 (11)||253||65 (26)|
|Present study||351||54 (15)||297||44 (15)|