What is ctDNA?

ctDNA is a unique kind of biomarker found in the blood. If you’ve been around a pet that sheds, think about how many stray hairs can accumulate. Even for a low-shedding pet, there’s usually evidence it’s been there. Tumors can work the same way: they shed microscopic particles into the bloodstream, and these particles can be picked up in a blood test. 

How to discover ctDNA

A ctDNA test looks for fragments of DNA in the blood that came from tumor cells. This is called a liquid biopsy. 

Impact on screening

There is an FDA-approved test called Shield that examines a patient’s blood for evidence of colorectal cancer. (Patients need to be average-risk, have no symptoms, and be age 45 or older to qualify for this test.)

Any positive test requires a follow-up colonoscopy for it to be a complete colorectal cancer screening.  

Impact on treatment

In clinical trial settings for all GI cancers, ctDNA is being used to evaluate several things:

• Aggressiveness of cancer: Helps determine the tumor burden and how the cancer is expected to behave.
• Effectiveness of a treatment: Can be used to identify if a patient’s cancer is responding or resisting a certain treatment. Can help make surgery decisions. 
• Minimal Residual Disease (MRD): Used after surgery to identify if all the cancer was removed. May also be used once treatment has been completed to monitor for if cancer has returned (a recurrence). This test can often pick up a recurrence, sometimes months, sooner than an imaging test like a CT scan. 
• Biomarker testing: In some cases, especially if it’s challenging to get enough solid tumor tissue, ctDNA tests may be performed to identify a patient’s biomarkers.

Many research studies are evaluating the use of ctDNA in several settings. There are false positives and false negatives with these tests, and follow-up conversations with your doctor are important. 

Talk to your doctor about how ctDNA testing may apply to you.