Blood Color from Left Colon Bleeding
Blood originating from the left colon (descending colon, sigmoid colon, or rectum) appears bright red in the stool.
Color Characteristics by Anatomical Location
The color of blood in stool directly correlates with the transit time through the gastrointestinal tract:
Bright red blood indicates a left-sided colonic or anorectal source because minimal time elapses between bleeding and evacuation, preventing significant degradation of hemoglobin 1, 2, 3
Blood from the rectum and sigmoid colon (the most distal portions of the left colon) typically presents as bright red blood on the surface of stool or coating the stool, rather than mixed throughout 3
When colonoscopy identifies the bleeding source in patients reporting bright red hematochezia, 60% of diverticular bleeding originates from the left colon 4
Distinguishing Left from Right Colon Bleeding
Right colon bleeding more commonly produces maroon-colored or dark red blood because the longer transit time allows partial degradation of hemoglobin 3, 5
The presence of bright red blood mixed throughout the stool (rather than just on the surface) suggests a more proximal source but can still originate from the left colon with brisk bleeding 3
Clinical Implications
Approximately 75% of diverticula are located in the left colon, yet when angiography is used for diagnosis, bleeding appears to originate more often from the right colon—this discrepancy reflects the lower sensitivity of angiography for detecting less-severe left-sided diverticular bleeding 4
In patients under 55 years presenting with bright red rectal bleeding, all serious lesions except one malignancy were located within 60 cm of the anus (left colon and rectum) 2