What percentage of patients with bright red blood per rectum have a upper GI source of bleeding?

In one small single center study, about 10%. Chances increased if patient had history of upper GI bleeding or was anemic.


Source

Byers, S. DO et al. "Incidence of occult upper gastrointestinal bleeding in patients presenting to the ED with hematochezia" American Journal of Emergency Medicine. 2007.