The rectosigmoid ratio is a measurement of the diameter of the rectum divided by that of the sigmoid colon during contrast enema. It is of particular use in the diagnosis of Hirschsprung disease.
Normal children have a rectum that is larger than the sigmoid (i.e. rectosigmoid ration >1). In fact the rectum has the largest diameter of the left side of the colon.
A rectosigmoid ratio (R/S) less than 1 suggests the diagnosis of short-segment Hirschsprung disease.