The FDA recently approved the first-ever blood test for colon cancer screening. The test, Epi Procolon, detects a DNA marker of colorectal cancer known as methylated Septin9. In the presence of colon cancer, this marker can be shed into the bloodstream and detected non-invasively.
This is promising development, particularly if it helps to increase colon cancer screening rates, which are still unacceptably low. But the test is far from perfect. In fact, it was twice rejected by the FDA amid concerns regarding its accuracy and whether it would truly increase colon cancer screening.
A few specifics:
In the pivotal study presented to the FDA, the Epi Procolon test was compared to colonoscopy in 1,500 adults undergoing colon cancer screening.
The overall sensitivity of the test was 68.2%. This means that if you have colon cancer, the test will detect it 68% of the time. Alternatively, it will miss 32% of colon cancers.
The overall specificity was 79%. This means that if you do not have colon cancer, the test will be appropriately negative 79% of the time. Alternatively, it will return positive 21% of the time in the absence of colon cancer (false positive rate).
Based on the overall prevalence of colon cancer in the general population, a positive Epi Procolon test is associated with actual colon cancer only 2% of the time (this is known as the positive predictive value). This means that 49 out of every 50 positive tests are false.
The test cannot detect colon polyps.
Overall, this test has clear limitations. It has an acceptable, albeit sub-optimal, sensitivity and specificity. It is not a substitute for colonoscopy, which is both a screening and prevention test, and is the gold standard (the sensitivity and specificity of colonoscopy for colon cancer are close to 100% in experienced hands). It is also not superior to stool-based screening tests, which are more specific, cheaper, and supported by long-term studies of effectiveness.
At this time, I would view the Epi Procolon test as a secondary screening test, reserved for people unwilling to undergo a colonoscopy, and perhaps uncomfortable with a stool-based screening test. The actual cost, insurance coverage, and ideal testing frequency remain to be determined.
-Christopher McGowan, MD, MSCR