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Sensitivity is the share of people with the disease a test catches; specificity is the share of healthy people it correctly clears. No test scores 100% on both, and for screening the trade-off decides how many false alarms you get.
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.
For someone offered the test, these are the numbers that describe what a result means in an NHS population: about 1 in 100 tests came back positive each year, between 46 and 58 in 100 positives were cancer, and a negative result left about 1 in 100 with an undetected cancer within the year. The test found between a quarter and a third of all cancers diagnosed in a screening round, and about half to two thirds of the 12 cancer types it was designed to find. Whether finding them this way reduces late-stage diagnoses is the primary question, and this paper says only that the answer was no; the primary-endpoint paper had not appeared on Europe PMC by 23 September 2026.
In people who already have symptoms the test is more often right when positive, because cancer is commoner, but it still misses most stage I cancers, so a negative result cannot cancel an urgent referral.
Shares DeeP-C, Cologuard Plus, Positive predictive value (PPV), Screening.
Shares ECLIPSE (Shield blood test), Cologuard Plus, Positive predictive value (PPV), Screening.
Shares ECLIPSE (Shield blood test), Cologuard Plus, SimpleScreen CRC, Screening.
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Shares PREEMPT CRC, SimpleScreen CRC.
Shares BLUE-C, DeeP-C, Cologuard Plus.
Shares BLUE-C, DeeP-C, Cologuard Plus.
Shares PREEMPT CRC, SimpleScreen CRC.