High Rate of Positive Fecal Occult Blood Test in Healthy Infants: A Nested Case-control Study.

Background and objectives: Guaiac fecal occult blood test (gFOBT) is often used to evaluate evidence of food protein-induced allergic proctocolitis (FPIAP) in children in primary care and gastroenterology settings; however, it has not been validated for this diagnosis, and little is known about the positivity rates in early infancy. In this study, we used samples from healthy asymptomatic infants aged two weeks to two months to evaluate the gFOBT positivity rate compared to those diagnosed with FPIAP. Methods: This was a nested case-control study. Frozen stool samples from infants aged two days to five months enrolled in the Gastrointestinal Microbiome and Allergic Proctocolitis study were evaluated using gFOBT (n = 123). The results were interpreted by three blinded staff members, including a trained clinical research coordinator, a pediatric gastroenterologist, and an experienced medical assistant. Additionally, the samples were analyzed using a quantitative fecal immunochemical test (FIT) for hemoglobin to compare with gFOBT results. Results: Eight percent of samples from the 100 healthy asymptomatic infants were gFOBT positive (11% when including positive and equivocal results). Seventy-four percent of samples from infants diagnosed with FPIAP were gFOBT positive. The interrater reliability of gFOBT interpretation was 81%. Of the healthy samples that yielded a positive gFOBT result, 50% also yielded a positive FIT result. Of the 23 FPIAP samples that yielded a positive gFOBT result, 29% yielded a positive FIT result. Conclusions: Healthy asymptomatic infants in early infancy were gFOBT positive up to 11% of the time. Caution should be used when interpreting gFOBT results in young infants in a diagnostic setting.