A new noninvasive screening test can detect most cases of colorectal cancer and also many precancerous polyps, potentially helping to sharply reduce the death toll from the disease, according to results of a study released on Thursday.

Still, the results fell short of investor expectations and even those of the company that developed the test, the Exact Sciences Corporation, sending its shares down about 20 percent in afternoon trading on Thursday.

In its news release about the study Thursday morning, Exact Sciences said its test detected 92 percent of the cancers picked up by colonoscopy, and 42 percent of potentially precancerous polyps. It had a false positive rate of 13 percent.

The test looks for alterations in human DNA found in a stool sample. The company contends that people will not find it off-putting to deposit a sample of their stool in the company’s collection apparatus and mail it to a laboratory.