Sensitivity and specificity

Sensitivity (true-positive rate) and specificity (true-negative rate) are properties of the test at a chosen cut-off and are usually estimated against a reference standard such as colonoscopy or biopsy in a study like ECLIPSE or BLUE-C. What a person experiences, the chance that a positive result is real, is the positive predictive value, which depends on how common the disease is: at a 1% cancer prevalence, a test with 99% specificity still generates about as many false positives as true positives. Screening tests trade sensitivity against specificity by moving the cut-off; a multi-cancer blood test set to high specificity (99.5% for Galleri) accepts low sensitivity for early-stage cancer. Sensitivity for precancerous lesions, not cancer, is what distinguishes tests that prevent colorectal cancer from tests that detect it.