Purpose The International Study Group of Liver Surgery (ISGLS) defined post-hepatectomy biliary leakage as drain/serum bilirubin ratio > 3 at day 3 or the interventional/surgical revision due to biliary peritonitis. We investigated the definition’s applicability. Methods A retrospective evaluation of all liver resections over a 6-year period was performed. ROC analyses were performed for drain/serum bilirubin ratios on days 1, 2, and 3 including grade A to C (analysis I) and grade B and C biliary leakages (analysis II) to test specific cutoff values. Results A total of 576 patients were included. One hundred nine (18.9%) postoperative bile leakages occurred (19.6% of the whole population grade A, 16.5% grade B/C). Areas under the curve (AUC) for analysis I were 0.841 (day 1), 0.846 (day 2), and 0.734 (day 3). The highest sensitivity (78% on day 1/77% on day 2) and specificity (78% on day 1/79% on day 2) in analysis I were obtained for a drain/serum bilirubin ratio of 2.0. AUCs for analysis II were similar: 0.788 (day 1), 0.791 (day 2), and 0.650 (day 3). The highest sensitivity (73% on day 1/71% on day 2) and specificity (74% on day 1/76% on day 2) in analysis II were detected for a drain/serum bilirubin ratio of 2.0 on postoperative day 2. Conclusion Biliary leakages should be defined if the drain/serum bilirubin ratio is > 2.0 on postoperative day 2.