Doctors discovered blood in George's stool on Saturday along with a drop in his red blood count, the Chicago Tribune says.
As a result, tests were performed to examine his colon.
The complication forced the cardinal to return to a liquid diet and has delayed his release from the intensive care unit at Loyola University Medical Center in Maywood.
A spokesperson for the Chicago archdiocese said the cardinal's vital signs were normal on Sunday and his blood counts were stable but he remained in intensive care for observation.
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