Blood Tests for Colon Cancer Screening Are Now Available
Yes, you can now order a blood test for colorectal cancer screening—the FDA-approved Shield test by Guardant is commercially available as of May 2024, but it should be reserved primarily for patients who refuse or cannot complete stool-based testing or colonoscopy, not as a first-line screening option. 1
Key Performance Characteristics
The Shield blood test detects cell-free DNA from colorectal lesions with the following performance:
- CRC sensitivity: 83.1% (lower for stage 1 disease at 55%)
- Specificity: 89.6%
- Advanced precancerous lesion (APL) sensitivity: only 13.2% 1
This poor APL detection is a critical limitation—it substantially reduces cancer prevention potential since you're missing most advanced adenomas that could be removed before becoming cancer.
When to Use Blood Testing
Appropriate scenarios:
- Patients who have declined both FIT and colonoscopy despite counseling
- Patients with barriers to stool collection or colonoscopy access
- As a tool to bring unscreened individuals into the screening fold
NOT appropriate for:
- Patients willing to do FIT or colonoscopy (these remain superior first-line options)
- Interval testing between colonoscopies
- Replacement of established screening in compliant patients 1
The Adherence Trade-Off
Blood tests do increase screening uptake by approximately 11-17.5 percentage points compared to reoffering stool tests alone in patients who previously declined screening 1, 2. However, this improved adherence comes at the cost of:
- Missing 17% of cancers (vs. 83% sensitivity)
- Missing 87% of advanced adenomas that could be prevented
- Lower sensitivity for early-stage disease when cure rates are highest 1
Clinical Algorithm
For average-risk screening:
First-line options (offer these first):
- Annual FIT
- Multitarget stool DNA every 1-3 years
- Colonoscopy every 10 years
Second-line option (only after patient declines above):
- Blood test (Shield) with unknown optimal interval
- Positive results require colonoscopy follow-up
Critical caveat: A negative blood test does NOT rule out significant pathology given the 13% APL sensitivity 1
Cost-Effectiveness Concerns
Blood tests meeting current CMS coverage criteria are not cost-effective unless:
- Priced below $125 per test
- Advanced adenoma sensitivity exceeds 40% (current tests are at 13%)
- Used in patients who would otherwise remain unscreened 3
At $500 per test with current performance, blood testing yields fewer quality-adjusted life years (83-116 per 1000) compared to annual FIT (125-163 per 1000) or colonoscopy (132-177 per 1000) 3.
Common Pitfalls to Avoid
- Don't offer blood tests to patients willing to do FIT or colonoscopy—you're giving them an inferior test
- Don't use blood tests for interval screening between colonoscopies—no data supports this 1
- Don't assume a negative blood test means the patient is "clear"—the miss rate for advanced adenomas is 87%
- Ensure colonoscopy follow-up for positive results—this remains a challenge even when blood tests are completed 4
The Bottom Line
Blood-based CRC screening should function as a safety net for the unscreened, not as a replacement for established first-line screening methods. The 2025 AGA guidance is clear that while these tests are now available and may improve participation, their performance characteristics—particularly the poor advanced adenoma detection—make them suboptimal compared to stool-based tests and colonoscopy 1. The older septin 9 test has even worse performance (48-64% CRC sensitivity) and is not recommended by major guidelines 5.
Reserve blood testing for patients who have definitively refused other options, counsel them about its limitations, and ensure robust systems for colonoscopy follow-up of positive results.