A potential error in the quantitation of fecal blood loss: concise communication.

Chromium-51-labeled red cells were used to quantitate fecal blood loss in a patient with chronic upper gastrointestinal hemorrhage. On Day 1, the stool guaiac was positive but the blood loss indicated by 51Cr was less than 1 cm3. Blood loss in the stool by 51Cr did not become significant until Day 3, when it measured 23 cm3. The failure to detect abnormal blood loss on Day 1, and probably on Day 2, appears to be due to a long intestinal transit time from a proximal bleeding site. The problem of slow intestinal transit is not uncommon and could lead to a false-negative study or falsely low estimates of fecal blood loss. This problem could be minimized by beginning stool collection on Day 3 or by delaying stool collection until the appearance in the stool of an oral nonabsorbable marker swallowed when the 51Cr-tagged red cells are injected.