Blood tests for colon cancer screening: what “not preferred” really means

In 2026, the American Cancer Society (ACS) clarified that blood tests for colorectal cancer screening are “not preferred”—meaning they’re not meant to replace the stool-based and visual screening tests most people are offered first. The practical reason is that screening only counts if abnormal results lead to the right follow-up (typically a colonoscopy when appropriate).

So if a blood test (for example, Shield) is offered, the key question isn’t just “Is it FDA-approved?” It’s whether your situation matches the limited role ACS describes—and what happens next if the result is positive.

Quick context: what changed in 2026

ACS’s 2026 colorectal cancer screening guidance keeps several options on the table, but it positions blood-based tests as an alternative only for a limited group—for example, people who decline or don’t complete preferred screening (like stool-based tests or direct visualization).

ACS also emphasized that positive screening results—whether from stool or blood-based screening—require timely colonoscopy follow-up to complete evaluation. In ACS framing, follow-up is ideally within about 6 months.

What “not preferred” means in everyday terms

When “not preferred” is heard without context, it can sound like “optional but fine.” ACS’s intent is more specific:

Preferred screening options come first (stool-based tests and visual exams of the colon/rectum).
Blood-based tests are for situations where a preferred option isn’t being used (such as when someone declines or doesn’t complete preferred screening).
Blood-based tests are not as effective as preferred options for finding precancerous changes—so choosing a blood test should come with a clear, fast plan for next steps if results are abnormal.

The American College of Gastroenterology (ACG) also warned that the “not preferred” wording can be misunderstood—potentially leading some people to choose a lower-performing option when a preferred screening option might still be feasible.

A short checklist to bring to your appointment

If your clinic offers a blood test, you can use these questions to confirm it fits the ACS “limited role” idea:

What preferred screening options am I being offered first? (Stool-based or a visual exam.)
Am I choosing the blood test because I declined or didn’t complete a preferred option?
If my result is positive, how quickly will colonoscopy be scheduled? (ACS frames timely follow-up—ideally within about 6 months.)
Who will coordinate follow-up? Ask how results will be communicated and who helps ensure the colonoscopy is booked.
Am I eligible based on the test’s intended use? For Shield, FDA describes limits around who it’s for and what it’s not meant to replace.

If the blood test is offered anyway: confirm the intended use

It’s reasonable to ask whether your clinic is using the test in the way guidance and regulation describe. FDA’s overview of Shield emphasizes, in broad terms, that it’s intended for average-risk colorectal cancer screening (for people in the appropriate screening age range) and not a replacement for diagnostic colonoscopy or surveillance colonoscopy in higher-risk situations.

Practical takeaway: the “not preferred” label is about both how well the test performs and matching the test to the right patient and purpose—including making sure follow-up happens quickly if the result is positive.

If the blood test is positive: what happens next

A positive result should not be treated as the end of screening. The next step is usually colonoscopy because it’s the procedure that evaluates the colon/rectum directly and can remove precancerous polyps when appropriate.

CDC explains that abnormal results from many colorectal screening pathways generally require colonoscopy to complete the evaluation.
ACS highlights that timely follow-up colonoscopy is essential for screening tests to meaningfully reduce risk.
FDA similarly frames positive Shield results as requiring diagnostic colonoscopy follow-up.

If you’re trying to reduce delays, ask for the follow-up timeline and confirm who books the colonoscopy appointment (and how quickly).

Bottom line

The 2026 ACS clarification doesn’t mean blood tests are “wrong.” It means they’re not the first choice for most people and are intended for a limited situation: when someone declines or doesn’t complete preferred stool-based or visual screening.

To make screening count, focus on two things:

Were preferred options offered first?
If results are positive, is colonoscopy follow-up scheduled promptly?

If you’re unsure whether a blood test fits your situation, bring these questions to your appointment—especially eligibility and next-step scheduling.

Key sources

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This article is for general informational purposes only and is not medical advice. Research findings can be early or incomplete, and health guidance can change. Always talk with a qualified healthcare professional about personal symptoms, diagnosis, medications, vaccines, screenings, or treatment decisions. If you think you may have a medical emergency, call emergency services right away.