Figure 1.

Bleeding in active UC is controlled by successful formation of fibrin on mucosal erosions. (A) Patients experiencing flares of UC often suffer from rectal bleeding, typically assessed by the partial Mayo score (0: none, 1: visible blood with stool (< 50 %), 2: visible blood with stool (> 50 %), 3: passing blood alone). Patients suffering from flares of UC underwent routine endoscopy. Representative endoscopic images of patients with various degrees of rectal bleeding are depicted. (B) Clinically observed rectal bleeding coincided with the enhanced presence of mucosal erosions on endoscopy as graded by an experienced endoscopist in a blinded fashion inspired by the Blackstone score (0: no visible erosions, 1: less than 10 erosions (< 5 mm in size) per 10 cm section, 2: more than 10 erosions (< 5 mm in size) per 10 cm section to 3: more than 10 erosions (< 5 mm in size) and ulcerations (> 5 mm in size) per 10 cm section). The section most strongly affected determined the grading result (Spearman’s rho: 0.38, * p < 0.05). additionally, the morphology of mucosal erosions was analysed and the frequency of either fibrin coverage or persistence of fresh blood or haematin was assessed in these images supported by digital image analysis. (C) Presence of erosions did not necessarily prompt rectal bleeding, rather complete coverage by fibrin on all mucosal erosions achieved haemostasis. Less efficient fibrin coverage was observed in patients suffering from rectal bleeding as assessed by partial Mayo score (spearman Rho: 0.73, **** p < 0.0001). (D) Persistent presence of blood or haematin on erosions was associated with clinical rectal bleeding (spearman Rho: 0.73, **** p < 0.0001). The study cohort included 36 patients suffering from active UC with varying degrees of rectal bleeding. UC, ulcerative colitis.