The PSA test is a blood measurement widely discussed as a prostate cancer test. It measures something related to the prostate but not specific to cancer, and that distinction drives every debate about how to use it.
The protein is made by the gland, not by the disease
Prostate specific antigen is an enzyme produced by prostate cells and secreted into semen, where it liquefies the ejaculate.
Small amounts leak into the bloodstream from normal tissue, and the test measures that circulating concentration.
It is specific to the prostate as an organ. It is not specific to cancer, which is the source of the confusion in its name.
Several ordinary things raise the level
Benign enlargement of the prostate, common with age, increases the amount of tissue producing the protein and therefore the blood level.
Inflammation or infection of the gland raises it, sometimes substantially, and so can recent ejaculation, vigorous cycling or a recent examination.
An elevated result therefore indicates that something is affecting the prostate, without indicating what.
Cancer can be present at a normal level
The relationship also fails in the other direction. Some prostate cancers produce relatively little of the protein, so the blood level stays within the usual range.
There is no threshold that cleanly separates men with cancer from men without it; the two distributions overlap.
Any cut-off chosen trades missed cancers against unnecessary investigation, and moving the line changes which error becomes more common.
Overdiagnosis is the harder problem
Many prostate cancers grow so slowly that they would never have produced symptoms within a man's lifetime.
Detecting these is not a benefit, because treatment carries real risks to urinary and sexual function while the untreated tumour would have caused no harm.
The screening debate turns on this point rather than on whether the test detects cancer, which it does.
How the result is used in practice
A single value carries limited information. Change over time, the size of the gland, age and family history all shape interpretation.
Further steps such as imaging and biopsy are how a raised result is resolved, and modern pathways use them to reduce unnecessary biopsies.
Whether to test at all is a decision to make with a doctor, weighing personal risk factors against the prospect of investigation that may find something better left alone.