Colonoscopies not the gold standard?
In addition, the biology of the colon is different on the two sides. Polyps on the left — which are reachable with a sigmoidoscope — have a stalk and look a bit like mushrooms, while polyps on the right are generally quite flat. This makes them harder to find and snip out.

But just because the test has its complications doesn't mean it isn't — still — a better test, says Dr. David Johnson, chief of gastroenterology at Eastern Virginia Medical School in Norfolk and past president of the American College of Gastroenterology.

"It would be a gross overreaction and inappropriate response to infer that colonoscopy does not provide a benefit," he says. "That would be taking us back 20 years, at a time when we're making great strides in reducing colon cancer mortality."

Others are skeptical that colonoscopy will ever prove to be more valuable than sigmoidoscopy.

"If we had very, very, very high-quality colonoscopies, we still don't know if we'd see a benefit on the right side," Rabeneck says.

The debate is likely to continue for some time, perhaps until results come in from two large randomized trials in Europe that aim to nail down the benefit of colonoscopy by comparing patients who get them to patients who get other types of screening tests or no screening at all. But those studies, which each involve more than 50,000 patients, will take at least a decade to complete.

In the meantime, experts on both sides of the issue emphasize that either test is much better than no test at all. In fact, they say, any of the available screening tests is worthwhile.

Take fecal occult blood tests, which check for blood in stool samples. Although they've been overlooked somewhat in the current debate, so far they're the only other tests, besides sigmoidoscopies, to have been proved effective in a randomized trial.

Fecal blood tests are relatively low-tech. They're also relatively cheap, convenient and non-risky. And some doctors and researchers believe they don't get enough respect.

"I could do more for the public good, if I wereczar, if I started doing stool blood tests on everybody tomorrow, following up with colonoscopies if need be," says Brawley of the American Cancer Society. "That would save more lives than what we're doing now."

Still, for Brawley, every test is a favorite test: "The best test is the test you decide to get."