Colonoscopy proved far superior to two other common tests for colon cancer in a study that could put pressure on more insurance companies to cover the expensive screening method.

Government researchers found that the two most widely used screening tests for colon cancer together missed one-quarter of the tumors and precancerous growths detected by colonoscopy.

Experts consider colonoscopy the “gold standard” test for the nation’s No. 2 cancer killer, but they have been trying to establish with more certainty which method really is the best. Medical guidelines generally call for annual testing for blood in a patient’s stool and, every five years, a sigmoidoscopy, or examination of the lower colon, both starting at age 50. Some guidelines recommend colonoscopy as a routine test; others recommend it only for people who run a high risk of colon cancer, such as those with a family history of it, those who have had previous growths, or those who show signs of cancer during the stool test or a sigmoidoscopy.