AKDOĞAN, Meral, et al. “Are Serum and Biliary Carcinoembryonic Antigen and Carbohydrate antigen19-9 Determinations Reliable for Differentiation Between Benign and Malignant Biliary Disease?”. Turkish Journal of Gastroenterology, vol. 14, no. 3, Jan. 2003, pp. 181-4, https://www.turkjgastroenterol.org/index.php/tjg/article/view/1265.