Cut-off value of pleural fluid C-reactive protein in etiologic diagnosis of pleural fluid

Classification of effusion into transudates or exudates is considered as the corner stone in the etiological diagnosis of pleural effusion. To determine the validity of pleural fluid (high sensitivity-CRP) concentration in etiologic diagnosis of pleural effusion and to obtain a cut-off value of pleural fluid CRP at which we can discriminate between exudative and transudative pleural effusions. A study was conducted upon a hundred patients with pleural effusion. All patients were subjected to: history, clinical examination, chest radiography and thoracic ultrasound, tuberculin test and aspiration of pleural fluid. The fluid was sent for biochemical examination including: Protein, L.D.H, A.D.A and CRP levels, cytological examination and bacteriologic examination. Classification of pleural fluid into transudative or exudative is based upon Light’s criteria. Data were compared by independent sample t-test for 2 groups or by a one-way analysis of variance (ANOVA) for more than 2 groups of variables. Simple correlations between variables were examined by calculating Pearson’s product correlation coefficient, Receiver operating characteristic (ROC) curve used to calculate cut off points, area under the curve (AUC), sensitivity and specificity, and 2 tailed P < 0.05 was considered significant. There were significant differences between both groups as regards serum, fluid levels and fluid/serum ratio for LDH, total protein and CRP except for CRP fluid/serum ratio. There was a significant correlation between CRP and LDH and total protein fluid levels. Receiver operating characteristic (ROC) curve was used to calculate the sensitivity and specificity of CRP fluid level and also the cut off value of CRP fluid level. Out of the 44 patients with exudative pleural effusion, two cases were diagnosed as cardiac effusion and one case as liver cirrhosis. The three cases were receiving diuretics and the pleural fluid analysis was repeated after withdrawal of the diuretics which turned to be transudative according to Light’s criteria. CRP could be a useful diagnostic marker for differentiation between exudative and transudative pleural effusions and also it is more accurate than protein in distinguishing those transudative effusions receiving diuretic therapy which are falsely diagnosed by Light’s criteria to be exudates.