Why screening uses two tests in sequence
A two-stage pipeline pairs a sensitive first test with a specific second test. The first stage casts a wide net so that few true cases slip through, accepting that it will also flag many people who do not have the disease. The second stage is more specific and is applied only to those who screened positive, weeding out most of the false positives from stage one.
Sequencing this way exploits the strengths of each test. A highly sensitive screen on its own would generate too many false alarms to act on; a highly specific confirmatory test applied to everyone would be too costly or invasive. Running them in series gives both reach and precision.
How the population funnels down
Picture the whole tested population entering at the top. The sensitive screen passes through nearly all true cases plus a large band of false positives. Confirmation then removes most of those false positives, leaving a narrow stream of confirmed cases. Each stage shrinks the group and raises the proportion that truly has the disease.
This is general educational information, not medical advice. Screening intervals, eligibility, and confirmatory pathways are set by clinical guidelines and should be discussed with a qualified professional.