Background and AimBlood markers are not always regarded as satisfactory surrogate biomarkers for predicting endoscopic activity in ulcerative colitis (UC). However, those biomarkers have been evaluated solely based on endoscopic activity at the most severe colorectal location, taking no account of the extent of inflammation. This study aimed to examine whether integrated evaluation of severity and extent of endoscopic activity improves the performance of blood biomarkers for UC.MethodsWe performed a retrospective study of UC patients who underwent colonoscopy and blood tests in our hospital. Blood tests were C‐reactive protein (CRP), serum albumin (ALB), and platelet count (PLT). We compared blood markers with two versions of endoscopic activity assessed by Mayo endoscopic subscore (MES): the maximum score of MES in the colorectum (mMES, range: 0–3) and the cumulative score of MES of six colorectal regions (cMES, range: 0–18).ResultsAll three blood markers correlated well with both mMES and cMES, and each marker showed better correlation with cMES than mMES (Spearman rank correlation coefficient: PLT: 0.54 vs 0.47, ALB: −0.65 vs −0.52, and CRP: 0.52 vs 0.38, respectively). The predictability, including sensitivity and specificity, of each marker for endoscopic activity was also better for cMES, resulting in higher degrees of area under the curve (mMES vs cMES: PLT: 0.75 vs 0.83, ALB: 0.77 vs 0.90, and CRP: 0.75 vs 0.90, respectively).ConclusionWhen incorporating the extent of inflammation, blood markers are better at predicting endoscopic activity of UC than previously considered and could be used as a reliable biomarker in clinical practice.