Title: Endoscopic retrograde cholangiopancreatography and carbohydrate antigen 19-9 in the differential diagnosis of biliary strictures.

INTRODUCTION Bile duct strictures include a wide spectrum of benign or malignant diseases. OBJECTIVE To determine the usefulness of endoscopic retrograde cholangiopancreatography (ERCP) and circulating carbohydrate antigen19-9 (CA 19-9) in the differential diagnosis of biliary strictures. METHOD We used an observational, prospective and cross-sectional study in 75 patients with biliary stricture diagnosed by ERCP between October 2018-January 2020. The variables were: type of biliary stricture diagnosed by ERCP, biliary cytology and CA 19-9 levels. For the statistical analysis, descriptive statistic was used according to the type of variable. The relationship among them was performed using Pearson's chi-square and Fisher's exact probability tests, assuming the differences as significant when p <0.05. The cut-off point for CA 19-9 was calculated using the ROC curve and the Cohen's Kappa index was used to measure concordance between diagnostic methods. RESULTS Cytology was positive in 51 (68%) patients with biliary stenosis. The mean age was 63 years. Acute cholangitis predominated in malignant strictures (93.7%). There was agreement between the cytology and the cut-off value calculated for CA 19-9 of 85.4U / ml, with Kappa agreement index = 0.332 (p = 0.004); as well as between ERCP and cytology with Kappa concordance index = 0.701 (p <0.001). CONCLUSIONS A serum CA 19-9 value higher than 85.4 U / ml is highly related to neoplastic biliary stenosis.