Last year, I concluded a commentary for this news organization on colorectal cancer (CRC) screening guidelines by stating that between stool-based tests, flexible sigmoidoscopy, and colonoscopy, “the best screening test is the test that gets done.” But should that maxim apply to the new blood-based screening test, Guardant Health Shield? This proprietary test, which costs $895 and is not generally covered by insurance, identifies alterations in cell-free DNA that are characteristic of CRC.
Shield’s test characteristics were recently evaluated in a prospective study of more than 10,000 adults aged 45-84 at average risk for CRC. The test had an 87.5% sensitivity for stage I, II, or III colorectal cancer but only a 13% sensitivity for advanced precancerous lesions. Test specificity was 89.6%, meaning that about 1 in 10 participants without CRC or advanced precancerous lesions on colonoscopy had a false-positive result.
Although the Shield blood test has a higher rate of false positives than the traditional fecal immunochemical test (FIT) and lower sensitivity and specificity than a multitarget stool DNA (FIT-DNA) test designed to improve on Cologuard, it meets the previously established criteria set forth by the Centers for Medicare & Medicaid Services (CMS) to be covered for Medicare beneficiaries at 3-year intervals, pending FDA approval.
A big concern, however, is that the availability of a blood test may cause patients who would have otherwise been screened with colonoscopy or stool tests to switch to the blood test. A cost-effectiveness analysis found that offering a blood test to patients who decline screening colonoscopy saves additional lives, but at the cost of more than $377,000 per life-year gained. Another study relying on three microsimulation models previously utilized by the US Preventive Services Task Force (USPSTF) found that annual FIT results in more life-years gained at substantially lower cost than blood-based screening every 3 years “even when uptake of blood-based screening was 20 percentage points higher than uptake of FIT.” As a result, a multidisciplinary expert panel concluded that blood-based screening should not substitute for established CRC screening tests, but instead be offered only to patients who decline those tests.
In practice, this will increase the complexity of the CRC screening conversations we have with patients. We will need to be clear that the blood test is not yet endorsed by the USPSTF or any major guideline group and is a second-line test that will miss most precancerous polyps. As with the stool tests, it is essential to emphasize that a positive result must be followed by diagnostic colonoscopy. To addend the cancer screening maxim I mentioned before, the blood test is not the best test for CRC, but it’s probably better than no test at all.
Dr. Lin is a family physician and associate director, Family Medicine Residency Program, Lancaster General Hospital, Lancaster, Pennsylvania. He blogs at Common Sense Family Doctor.
A version of this article appeared on Medscape.com.