Background and AimsDiagnostic performance of ascitic fluid total protein (AFTP) concentration remained unsettled. Our aim was to determine diagnostic value of AFTP in differential diagnosis of causes of ascites. MethodsSeven hundred four consecutive patients with new-onset ascites were prospectively enrolled in this study. ResultsIn the training cohort, diagnostic performance of quantitative AFTP assay was superior to that of Rivalta test in differential diagnosis of ascites. At the predetermined cut-off value of 25 g/L, quantitative AFTP assay was more useful in the differentiation of non-portal hypertensive ascites from portal hypertensive ascites compared with the exudate-transudate classification, area under curve of receiver operating characteristic curve was 0.958. Quantitative AFTP assay was superior to serum-ascites albumin gradient in the detection of non-portal hypertensive ascites, especially malignant ascites and tuberculous peritonitis. In mixed ascites, AFTP was useful in identifying peritoneal lesions. ConclusionsAscitic fluid total protein is a useful marker in non-portal hypertensive ascites; thus, it should be determined in diagnostic work-up of the patients with ascites.