In the neonatal period, the most likely cause of lower gastrointestinal bleeding is swallowed maternal blood, either from delivery or cracked nipples during breast-feeding. The Apt test differentiates fetal from maternal blood. The blood in question is mixed with alkali to detect conversion of oxyhemoglobin to hematin. Fetal hemoglobin is more resistant to denaturation than adult hemoglobin. If the supernatant stays pink after addition of alkali, the blood is fetal in origin (a positive test). This should not be confused with the Kleihauer-Betke test, used to detect fetal-maternal hemorrhage in the pregnant female.
Necrotizing enterocolitis (C) is a complication of premature infants and presents with abdominal distension, bloody stools, and feeding intolerance. Milk protein allergy (B) should be suspected after introduction of a new formula. Infants who are breastfed are not immune to this condition if mom consumes cow’s milk in her diet. Since the mother in this vignette is a vegetarian and is breast-feeding, it is unlikely that a milk protein allergy is responsible for the patient’s guaiac positive stool. In older infants with painless hematochezia, congenital malformations such as small intestine duplication and Meckel’s diverticulum (A) should be considered. Meckel’s diverticulum is most common at 2 years of age.
Kharbanda AB, Sawaya RD: Acute Abdominal Pain in Children, in Tintinalli JE, Stapczynski JS, Ma OJ, et al (eds): Tintinalli’s Emergency Medicine, ed 7. New York, The McGraw-Hill Companies Inc., 2011, (Ch) 124:p 839-854