It's a question of false positives and false negatives. Most medical tests have a decent percentage of both.
> Screening test results may appear to be abnormal even though no cancer is present. A false-positive test result (one that shows there is cancer when there really isn't) can cause anxiety and is usually followed by more tests (such as biopsy), which also have risks. [1]
> Screening test results may appear to be abnormal even though no cancer is present. A false-positive test result (one that shows there is cancer when there really isn't) can cause anxiety and is usually followed by more tests (such as biopsy), which also have risks. [1]
[1] https://www.cancer.gov/types/colorectal/patient/colorectal-s...