Separated specimens were collected in tubes containing ethylenediamine tetra-acetic acid, centrifuged at 1,500g for 15 min at 4°C, and the superna-tants were stored at - 70°C until TNF-a analysis. Only the results of the first thoracentesis were considered.
All biochemical measurements were performed on a selective, discrete, multichannel analyzer (Hitachi 717 and 917; Hitachi; Tokyo, Japan) using standard methodology. Specifically, pleural ADA activity and pH were assessed with an automated ultraviolet kinetic test (Roche Diagnostics; Barcelona, Spain) and through a blood-gas machine, respectively. Cellular counting was performed in a Thoma chamber (Weber Scientific International; Middlesex, UK). Human TNF-a in pleural fluid was measured by an immunoenzymometric assay (Biosource Europe, S.A; Nivelles, Belgium), according to the instructions of the manufacturer. The investigator running the TNF-a assay was blinded of clinical information, and similarly clinicians who made the decision to institute pleural drainage did not know the results of the cytokine test.
Statistical Analysis
Continuous data are reported as medians (quartiles). The x2 and Kruskal-Wallis tests were used to compare groups for qualitative and quantitative variables, respectively. To compare performances of biochemical pleural fluid analytes, receiver operating characteristic curves were constructed. The cutoff value for TNF-a with the highest diagnostic accuracy was selected in order to discriminate UPPE from CPPE. To identify predictors of CPPE, we used a backward conditional stepwise logistic regression analysis on dichotomized biochemical variables of pleural fluid.
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