Although recent studies have shown that YKL-40 serum levels could be utilized in the diagnosis of endometrial carcinoma (with 74 % sensitivity and 87 % specificity) [32], and of esophageal squamous cell carcinomas (with 73 % sensitivity and 84 % specificity) [33], or could provide information regarding the response to chemotherapy and overall survival in patients with small cell lung cancer [34], this research provides evidence for the first time that YKL-40 could also be used for the differential diagnosis of tuberculosis from other pleural effusions. This evidence concerns the gene CHI3L1 and small cell lung carcinoma.