Which of the following statements about diagnostic studies for the colon and rectum is/are true?

B. Cinedefecography is useful for detecting “hidden” prolapse or rectal intussusception.

C. A negative osmotic gap in stool is indicative of secretory diarrhea

D. A colonic transit time study involves serial abdominal x-rays after ingestion of radiopaque markers.

E. Carcinoembryonic antigen (CEA) is useful for monitoring patients after resection for colon cancer.

DISCUSSION: Rectal biopsy should be performed in all patients with Hirschsprung's disease. The biposy specimen is examined for ganglion cells, but a much more reliable method is staining acetylcholinesterase, which has an accuracy rate of 99%. Cinedefecography involves instilling contrast medium into the rectum and recording the act of defecation with static radiographs and videofluoroscopy videotape. While the static radiographs allow measurement of the anorectal angle and the relationship between the anus and the coccyx/pubis plane, the videotape is used to detect rectal intussusception, which can be missed in static radiographs. In equivocal cases differentiation between malabsorptive and secretory diarrhea can be made by measurement of electrolytes and osmolality in stool. The osmotic gap is calculated by subtracting 2 × (Na + K) from the measured osmolality. A negative osmotic gap is indicative of secretory diarrhea, whereas a positive osmotic gap indicates malabsorptive diarrhea. In a colonic transit study, 20 radiopaque barium-impregnated markers are ingested before breakfast and plain films of the abdomen are obtained on days 4 and 6. The value of CEA is controversial, but most surgeons agree that it is helpful in monitoring following resection for colon cancer.