Cancer Screening

Screening looks for cancer or a cancer precursor before symptoms appear. Its benefit depends on more than finding disease: the condition must have a detectable preclinical phase, the test must perform well enough, and earlier treatment must improve outcomes rather than merely create lead-time bias. Colorectal screening illustrates both mechanisms: some tests detect occult blood or altered stool DNA, while colonoscopy can identify and remove precancerous lesions. The first clinical question is therefore not, “Which screening test is due?” It is, “Is this person asymptomatic and at average risk?” USPSTF recommendations are written for that population. A patient with unexplained iron-deficiency anaemia, rectal bleeding, a breast lump, or postcoital bleeding needs diagnostic evaluation. A normal screening interval does not make those findings safe to defer. A positive screening result is a signal, not a cancer diagnosis. False-positive results lead to additional imaging, biopsy, procedures, anxiety, and sometimes treatment. False-negative results can falsely reassure. Positive predictive value also changes with disease prevalence: the same test can produce more false alarms when used in a population with very little disease. Known BRCA1/2,...