What Does Tumor-Informed ctDNA Mean?

Circulating tumor DNA, or ctDNA, is DNA that a tumor sheds into the bloodstream. A blood draw can pick it up. The hard part is telling it apart from normal DNA. Normal DNA is in the same tube, in far larger amounts.

There are two ways to solve that, and the label on your report tells you which was used.

A tumor-informed test starts with your tumor. The lab sequences tissue from your surgery or biopsy. It finds mutations unique to that tumor. Then it builds a blood test that hunts only for those mutations. The panel is made for you and works for no one else.

A tumor-naive test, also called tumor-agnostic or plasma-only, skips the tissue step. It uses one fixed panel for everybody. That panel reads regions that are often mutated. It also reads chemical tags on DNA, called methylation marks.

Take Signatera as the worked example, since it is the most studied. The lab runs whole exome sequencing on the tumor tissue. That means it reads the protein-coding parts of the genome. From that read, it picks 16 somatic variants. Somatic means the tumor acquired them. They were not inherited.

Those 16 targets become your personal blood assay. The reported limit of detection is 0.01 percent variant allele frequency. That means the test can flag a mutation present in about 1 of every 10,000 DNA fragments. A sample is called positive when at least 2 of the tracked variants show up.

Other tumor-informed assays include FoundationOne Tracker, MRDetect, and Safe-SeqS. On the tumor-naive side, Guardant Reveal reads more than 1,000 genomic regions and more than 2,000 methylation sites, without needing tissue.

A ctDNA result only means something at the right moment. Surgery itself releases DNA into the blood. Draw too early and that noise can muddy the result.

Published performance figures come with their own draw windows:

• Signatera, about 30 days after surgery: 88 percent sensitivity, 98 percent specificity
• MRDetect, median 43 days: 84 percent sensitivity, 99 percent specificity
• FoundationOne Tracker, median 26.5 days: 69 percent sensitivity, 100 percent specificity
• Safe-SeqS, 4 to 10 weeks: 48 percent sensitivity, 98 percent specificity
• Guardant Reveal, 11 to 148 days: 91 percent sensitivity, 100 percent specificity

Sensitivity is the share of people with leftover cancer that the test correctly flags. Specificity is the share of cancer-free people it correctly clears. Notice the pattern: specificity is very high across the board, sensitivity is not. A positive result is hard to dismiss. A negative result is softer news than it sounds.

The BESPOKE CRC study followed people with colorectal cancer after surgery. It began in 2020 and enrolled nearly 1,800 people. Results from 623 of them were presented in 2024. Natera, the company that makes the test, paid for the study.

Testing ran on a schedule: 1 month after surgery, every 3 months for the next year, then every 6 months.

Of the 623 participants, 381 received chemotherapy after surgery. Among those, 85 had a positive ctDNA result and 296 had a negative one.

The gap in outcomes was wide. People with positive ctDNA who got chemotherapy had a median recurrence-free survival of almost 18 months. People with positive ctDNA who did not get chemotherapy had about 7 months. Now the negative group. More than 90 percent went a median of over 2 years with no recurrence. That held whether or not they had chemotherapy.

Of the more than 500 people whose first test was negative, only 14 later turned positive.

ctDNA usually turns positive before a scan shows anything. A review of colorectal studies puts the median lead time over imaging at 8.7 months, with a range of 0.8 to 16.5 months. In BESPOKE, a substantial number of the metastases found during follow-up were picked up first by ctDNA testing.

Knowing earlier is not automatically the same as living longer. That is the exact question the current trials are built to answer. In BESPOKE, 101 patients had their cancer come back. Of those, 40 had oligometastatic disease. That means a small number of spots, few enough that surgery or targeted radiation may still work. Finding them earlier could matter. It has not been proven that it does.

Read the sensitivity column again. A test that catches 88 percent of true residual disease misses roughly 1 in 8. Very small deposits of cancer may shed too little DNA to detect. Some tumors shed poorly no matter their size.

The background risk also stays. NCI notes that about a quarter of people with stage II or III colorectal cancer are not cured, even with chemotherapy after surgery. A negative ctDNA test lowers your estimated risk. It does not zero it, and it does not replace your scan schedule unless your oncologist says so.

In November 2024 the FDA issued guidance titled "Use of Circulating Tumor Deoxyribonucleic Acid for Early-Stage Solid Tumor Drug Development." It treats ctDNA as a biomarker in early-stage disease, where the goal is cure. Its focus is testing for molecular residual disease. It also pushes for assays and methods that are standard across labs.

That framing is worth noticing. The document is about how to use ctDNA to develop drugs. It is not a statement that ctDNA results should drive individual treatment decisions today.

• Is my test tumor-informed or tumor-naive, and which product is it?
• How many days after surgery was the blood drawn, and does that match the validated window?
• If the result is positive, what specifically changes? More chemotherapy, different chemotherapy, or earlier imaging?
• If the result is negative, does my scan schedule change at all?
• Is my insurance covering this, and what is my share if it does not?
• Is there a clinical trial where this result would guide treatment under a protocol?

https://www.cancer.gov/news-events/cancer-currents-blog/2024/colorectal-cancer-ctdna-may-guide-adjuvant-therapy

https://www.fda.gov/regulatory-information/search-fda-guidance-documents/use-circulating-tumor-deoxyribonucleic-acid-early-stage-solid-tumor-drug-development-guidance

https://pmc.ncbi.nlm.nih.gov/articles/PMC12186373/