The Problem with Screening

The problem with screening for colon cancer with colonoscopy, breast cancer with mammography, cervical cancer with Pap smears, etc. is that the positive predictive value of such tests is highly dependent on prevalence of diseases. Most of abnormal findings on mammography are false positives; the screen detected something that turned out not to be cancer. Obviously, the predictive value of a test increases if the prevalence of the disease increases. The problem is the highest prevalence of these cancers are seen in those with low socioeconomic status. Yet patients with low socioeconomic status are not the ones getting screened. The problem with our health insurance system now is that it systematically excludes from screening those who would benefit the most from it.