Blood in stool looks different depending on how early it enters the digestive tract—and thus how much digestive action it has been exposed to—and how much there is. The term can refer either to melena, with a black appearance, typically originating from upper gastrointestinal bleeding; or to hematochezia, with a red color, typically originating from lower gastrointestinal bleeding. Evaluation of the blood found in stool depends on its characteristics, in terms of color, quantity and other features, which can point to its source, but more serious conditions can present with a mixed picture, or with the form of bleeding that is found in another section of the tract. The term "blood in stool" is usually only used to describe visible blood, and not fecal occult blood, which is found only after physical examination and chemical laboratory testing.
In infants, the Apt test, a test that is particularly useful in cases where a newborn has blood in stool or vomit, can be used to distinguish fetal hemoglobin from maternal blood based on the differences in composition of fetal hemoglobin as compared to the hemoglobin found in adults. A non-harmful cause of neonatal bleeding include swallowed maternal blood during birth; However, serious causes include Necrotizing Enterocolitis (NEC), a severe inflammatory condition affecting premature infants, and midgut volvulus, a life-threatening twisting that requires emergency surgery.
