I know that an abdominal or pelvic x-ray has limitations. but would it detect a large blockage or a very big macroscopic polyp?

X-ray can help: Hello Sean, If there is a polyp large enough to cause an obstruction or large blockage then yes, an X-ray could definitely help in making the diagnosis. Feces can be seen on a 'Scout film of the abdomen' above the blockage. The doctor may look for Air-Fluid levels. These are diagnostic of an ileus because the bowel can not pass feces, air or liquids in an effective manner.

Not polyp, maybe blo: An abdominal X-ray wouldn't detect even a big polyp, as the polyp's density is roughly the same as the surrounding colon. A (large) blockage of the intestine / colon, called a small / large bowel obstruction (SBO / LBO), cd b detected if enuf air accumulates b4 the blockage to show a difference in density (air to colon). However, hard stool, lymph nodes, & scarring cd cause a SBO aside from polyps