Blood test to detect colon cancer gains traction, radiologists remain unconcerned

Despite the increasing popularity of blood testing for colorectal cancer, radiologists don’t have to worry CT colonography will be replaced just yet, according to experts.

A new test from Epigenomics and Warnex Medical Laboratories is infiltrating the ranks of the colon cancer screening world. On Dec. 6 the companies announced the Canadian launch of their diagnostic blood testing service. The test is derived from a blood sample and detects cell-free methylated DNA of the Septin9 gene shed into the bloodstream by colorectal tumors.

Diagnostic Imaging first reported on the Septin9 test in October. But researchers remain unconvinced Septin9 will edge out CT colonography.

“The Septin9 blood test is meant to identify colorectal cancer after it has developed and potentially already spread outside of the bowel wall,” said Dr. Judy Yee, a professor and vice chair of radiology and biomedical imaging at the University of California, San Francisco and a CT colonography expert. “It is not meant to prevent colorectal cancer.”

The purpose of CT colonography, on the other hand, is to identify precursor polyps before cancer develops, she said.

“The goals of the two tests are very different,” she said.

The Septin9 blood test may not be mature enough yet for use, according to Dr. Perry Pickhardt, a professor of radiology at the University of Wisconsin, Madison and also a CT colonography expert.

In the PRESEPT study, Septin9 detected two-thirds of colorectal cancers with a specificity of only 88%.

That means one in every three cancers would be missed, Pickhardt said.

“If this were applied to a screening population, where cancer is present in about one in every 500 adults, there would be more than 60 false-positive tests for every cancer detected,” Pickhardt said. “I don’t believe these are acceptable results, especially when you compare other noninvasive options, such as CT colonography, where the sensitivity and specificity for cancer detection is >95%.”

An attractive option might be following the Septin9 blood test with CT colonography because the risk of cancer is too low to justify colonoscopy in all cases, he said.

In any case, CT colonography remains viable in the radiology world for the detection of colon cancer.