Why a raised PSA is an ambiguous signal
Prostate-specific antigen is a protein made by prostate tissue, and its level in blood tends to rise with prostate cancer. The problem is specificity: PSA also rises with benign enlargement, inflammation, infection, recent ejaculation, and even bicycle riding. So while the test is reasonably good at not missing cancer, a raised value frequently reflects something benign, producing many false alarms that lead to further testing and biopsy.
This is the core tradeoff the diagram captures. Higher sensitivity catches more true cancers but the poor specificity means a positive result often does not mean cancer, especially at borderline levels.
Overdiagnosis as the central harm
The more subtle harm is overdiagnosis: PSA screening can detect slow-growing cancers that would never have caused symptoms or shortened life, yet their detection can trigger treatment with real side effects such as incontinence and erectile dysfunction. This is why screening guidelines increasingly favour shared decision-making rather than blanket testing, weighing a modest reduction in prostate-cancer death against the burden of overdiagnosis.
This is general educational information, not medical advice. Whether and when to have a PSA test is a personal decision best made with a clinician.