Can colon cancer be detected with a blood test? Why US FDA cleared this screening method

Screening for colorectal cancer is essential because it improves the odds of survival immensely. Studies show the five-year survival for colorectal cancer, when detected at an early stage, is 90 per cent as compared to the 15 per cent on metastasis.

How does the test work?

The Shield test is based on next-generation sequencing, by detecting certain genetic and epigenetic changes in the DNA present in the blood. Epigenetic changes are those that do not alter the genetic sequence but change what they do.

Physicians simply have to draw a blood sample for the test. Then, one of the components of blood called plasma is separated and enriched for specific genes that are released by cancerous cells.  Patients whose reports indicate “abnormal signal detected” may have cancer or non-cancerous tumours. They need to undergo further evaluation.

Can it replace colonoscopy?

Those found to have abnormalities will still need to undergo colonoscopy to confirm a cancer diagnosis. Those with a high-risk of colorectal cancer or family history should anyway go for regular colonoscopy.

What is the benefit of the new test?

Drawing blood is much more pleasant than alternatives. While a stool test is associated with an unpleasantness of handling, colonoscopy may be uncomfortable for many. This is fast and convenient.

Who is the blood test recommended for?

The Shield test can be first-line of screening in adults over the age of 45 years with average risk of colorectal cancers.

What do doctors say about the practical application of the test? 

While the test has been approved by the FDA, it has not been recommended as part of the standardised guidelines by professional bodies. “The clinical use of several biomarker tests such as this is limited at the moment. This is because proper guidelines have to be developed on what the next steps should be. More importantly, it has to be seen whether the test actually results in any mortality benefit at the community level,” said Dr Shankar.

What does the study say?

The FDA approval was based on the results of the ECLIPSE trial of over 20,000 people. It showed that the test could pick up 83 per cent of actual cancer cases. “This performance is within range of current guideline-recommended non-invasive screening methods, in which overall colorectal cancer sensitivity ranges from 74 per cent to 92 per cent,” the company said in a statement.