In differential diagnosis of pleural effusions, cytology is the most sensitive method. Since cytology findings are positive in half of such fluids, combined use of reliable tumor marker and cytology is a logical approach. Sialic acid and other tumor
markers can be reliable substances associated with neoplasia. The present study measured sialic acid levels in pleural effusion and serum samples of patients with malignant and nonmalignant diseases to discriminate each other, and to diagnose malignant effusion in a simple, cheap and reliable way. Sixty patients with pleural effusion were enrolled in the study and classified into two groups, group (I) 30 patients with malignant pleural effusion, and group (II) 30 patients
with nonmalignant pleural effusion. Pleural fluid (PF) and serum (S) levels of sialic acid were measured prior to any therapy; and PF/S sialic acid ratios were calculated. Pleural fluid and serum levels of sialic acid were significantly higher in malignant group compared to nonmalignant one. PF/S sialic acid ratio was higher in malignant group compared to nonmalignant group. In the malignant pleural effusion group, smokers showed a statistically significant higher pleural fluid and serum sialic acid levels as compared to nonsmokers. By using ROC curve, the cut off value of malignant pleural fluid sialic acid was 69.65 mg/dL, sensitivity was 70%, and specificity was 96%.