Screening means testing people who have no symptoms of a disease to look for hidden, early evidence of the disease. Many tests can be used for screening, including mammography for breast cancer, colonoscopy for colon cancer, and PSA testing for prostate cancer.

How a test is used determines whether it is a screening test. When a woman with no signs or symptoms of breast cancer gets an annual mammogram, she is getting a screening test. But when a woman feels a lump in her breast and gets a mammogram, she is getting a diagnostic test in response to a symptom, not a screening test.