Non-invasive DNA test picks up more colon cancers and more false positives

A stool-sample DNA test has detected 92 percent of colon cancers in a large clinical trial, compared to 74 percent picked up by the most commonly used non-invasive test.

But the test also had a greater propensity to detect cancer in normal samples ('false positives') leading some commentators to question its usefulness.

Study author Professor Thomas Imperiale, of the Indiana University Medical Centre in the USA, maintained that 'the most important finding of the study is the [test's] high sensitivity for curable stage colorectal cancer, which represents the highest sensitivity of any non-invasive test to date'.

'It is also significant to note that these results were achieved in a robustly conducted multicenter study', he said.

The trial involved nearly 10,000 men and women between 50 and 85 years old who had not been diagnosed with colon cancer and had a normal risk of developing it. The test, called Cologuard, checks samples for patterns in DNA associated with cancer or pre-cancer. No bowel preparation, medication restriction or dietary changes are required prior to sample collection.

The most-used non-invasive test in the USA, the fecal immunochemical test (FIT) picked up around 20 percent fewer cancers than the DNA test.

Cologuard also detected 42 percent of polyps, growths which can turn into cancer over several years, as opposed to 24 percent detected with FIT. Colonoscopy, which is presumed to find all colon cancers, was used as the reference method and detected 65 colorectal cancers and 757 pre-cancerous lesions in all.

Dr Kenneth Wang, president of the American Society for Gastrointestinal Endoscopy, who was not involved in the study, said: 'This test has the potential to bring colon cancer screening to many patients who might otherwise go without any screening'.

'I'm hopeful that this will increase the number of patients obtaining lifesaving colonoscopy with early detection and removal of precancerous and early cancerous polyps'.

However, some think Cologuard's higher false positive rate may prove a sticking point. Around 13 percent of subjects using the DNA test erroneously tested positive for potentially cancerous polyps, compared with around five percent of subjects using the FIT.

'The higher the false-positive rate, the less you want to apply the test', Dr Douglas Robertson, a gastroenterologist at the Geisel School of Medicine at Dartmouth told the Wall Street Journal. 'Otherwise, you might as well just get a colonoscopy'.

Professor Imperiale, however, said that 'all screening tests for low-prevalence conditions such as cancers [...] will have more false-positive test results than true-positive test results'.

Cologuard is due to be evaluated by the US Food and Drug Administration with an approval decision expected this summer.