The Colon Cancer Blood Test: What It Finds, What It Misses
Bloating & DigestionArticleA blood draw for colon cancer screening sounds easier than a colonoscopy or stool kit. In its main trial, one cell-free DNA test caught 83% of cancers but only 13% of advanced precancerous growths, and about 1 in 10 people without advanced findings got a false alarm. A modeling study suggests yearly blood testing may roughly match stool tests if people keep up with it.
In this article
More than a third of people who should be screened for colon cancer aren’t up to date.1 A blood test that looks for tumor DNA fragments (cell-free DNA) is meant to reach some of them. Here’s how one such test performed, and how it compares.
What the main trial found
About 7,900 average-risk adults had both the blood test and a colonoscopy.1
| What the colonoscopy found | Blood test caught it |
|---|---|
| Colon cancer (any stage) | 83% |
| Stage I to III cancer | 88% |
| Advanced precancerous growths | 13% |
About 10% of people without advanced findings had a positive blood test, a false alarm leading to a colonoscopy.1
The key limitation: colonoscopy and some stool tests aim to find and remove precancerous growths before they turn into cancer. This blood test mostly detects cancer that’s already there.12
How it stacks up over a lifetime
Researchers used the model behind federal screening recommendations to compare strategies, assuming everyone followed through perfectly.2
- Colonoscopy gave the most benefit.2
- Stool tests gave 81% to 88% of colonoscopy’s life-years gained.2
- Yearly blood tests gave 85% to 87%.2
- Blood tests every 2 or 3 years gave 57% to 72%.2
Two of the modeling study’s authors work for a company developing a blood test.2 The main trial was also funded by the test’s maker.
What this means for you
- If you’re willing to do a colonoscopy or stool test, those remain the stronger choices for prevention. See our guide to stool tests vs. colonoscopy coverage.
- If you won’t do either, a blood test is better than no screening, but it must be repeated on schedule.
- A positive result means a colonoscopy. Ask how it will be billed.
- It isn’t for symptoms or for people at higher risk, such as a strong family history. The trial studied average-risk adults.1
Our articles on colonoscopy prep and colonoscopy after 75 cover related decisions.
Questions to ask
- Am I average risk, or do I need a colonoscopy?
- How often would I repeat the blood test?
- Will my plan cover the test and the follow-up colonoscopy if it’s positive?
Try: “I don’t want a colonoscopy or a stool kit. If I choose the blood test, how often would I need it, and what happens if it’s positive?”
Bloated by Dinner includes a Belly Clock log and a Test-It list with a visit script. Use them to note any bowel changes and your screening history before you choose a test. See the book.
This article is for general educational purposes only and is not medical advice. It isn't a substitute for diagnosis or treatment from a qualified healthcare professional who knows your health history. Please don't skip a recommended colonoscopy because of something you read here. Blood in the stool, black stools, a change in bowel habits, belly pain, unexplained weight loss or anemia need medical evaluation, not a screening test. If you think you may be having a medical emergency, call 911 or your local emergency number right away. This article discusses screening research. It does not evaluate any particular company's test. Girl Playbook does not endorse any specific product, test, or provider.
Girl Playbook sells this book. It provides education and worksheets, not individualized medical care.
Sources
The research behind this article, in the order it’s cited. How we source our articles.
- New England Journal of Medicine (abstract via PubMed), 2024A Cell-free DNA Blood-Based Test for Colorectal Cancer Screening (opens in a new tab)abcde
- Journal of the National Cancer Institute (full text via PubMed Central), 2026Comparative benefits, burdens, and harms of emerging blood-based tests for colorectal cancer screening (opens in a new tab)abcdefg
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