PSA blood test: what prostate-specific antigen measures, normal PSA levels by age, what raises PSA besides cancer, and what happens after a high result

Prostate-specific antigen is an enzyme produced by the cells lining the prostate gland; its normal job is to liquefy semen. A small amount crosses into the bloodstream, and the PSA test measures that concentration in nanograms per milliliter (ng/mL). Anything that increases the amount of prostate tissue, disrupts the barrier between the gland and its blood supply, or irritates the gland raises the blood level — which is why PSA is specific to the prostate but not to prostate cancer.

Total PSA counts all of the protein in blood, whether free or bound to carrier proteins. Free PSA is the unbound fraction, reported as a percentage of the total; cancer tends to produce proportionally more bound PSA, so a low free percentage raises suspicion when total PSA is in the gray zone. PSA density (PSA divided by prostate volume on imaging) and PSA velocity (the rate of rise over time) are further refinements clinicians use before deciding on a biopsy.

PSA rises with age because the prostate slowly enlarges, so a single cutoff for all men either misses cancers in younger men or flags harmless enlargement in older ones. The traditional threshold of 4.0 ng/mL is still widely quoted; many laboratories and the American Urological Association also use age-specific upper limits.

Most elevated PSA results are not cancer. Among men with a PSA between 4 and 10 ng/mL, roughly one in four has cancer on biopsy; the rest have one of the following, listed from most to least common:

PSA screening is a genuine trade-off: it lowers the chance of dying from prostate cancer modestly, and it leads to biopsies and treatment for cancers that would never have caused harm. The US Preventive Services Task Force therefore recommends that men aged 55 to 69 make an individual decision after discussing the benefits and harms with a clinician, and recommends against screening men 70 and older. The American Urological Association's 2023 guideline supports shared decision-making from age 50 for average-risk men, from 40–45 for men at higher risk (Black men, a family history of prostate cancer, or known BRCA mutations), with re-testing every two to four years rather than annually for most men.

Both bodies emphasize that a single elevated PSA should be repeated before any further step, that a baseline in a man's forties is informative, and that men with a life expectancy under ten years are unlikely to benefit from screening. Men who are considering the test on their own can reasonably get a baseline; the decision about what to do with the number is the conversation worth having with a clinician.

The first step is almost always a repeat PSA after a few weeks, avoiding ejaculation for 48 hours and cycling for a few days beforehand, and after treating any urinary infection. A substantial fraction of elevated results return to normal on repeat. If the level stays high, the next steps depend on how high and on risk factors: a free-PSA percentage (values under about 10% raise suspicion, above 25% lower it), a PSA density if the prostate size is known, and increasingly a multiparametric MRI of the prostate before any biopsy — which lets many men with a normal MRI avoid biopsy altogether and targets the biopsy in those who need one.

Additional blood or urine markers (such as the 4Kscore, PHI or PCA3) are sometimes used in the 4–10 ng/mL range to refine the decision. A biopsy, when done, is what diagnoses cancer; PSA never does on its own. Men already diagnosed use PSA differently — as a treatment-response and recurrence marker, where the trend and the doubling time matter more than any absolute value.

PSA is a simple blood draw with no fasting, available at Quest and Labcorp, ordered by a clinician or directly through TestWell with physician review included; results post within a few business days. To avoid a falsely high reading, do not ejaculate for 48 hours before the draw, avoid vigorous cycling for a few days, and postpone the test if you have symptoms of a urinary or prostate infection or have had a recent rectal exam, catheter or prostate procedure. Tell the clinician if you take finasteride or dutasteride, which halve the result.

If you are having PSA for the first time, consider pairing it with the free PSA so the ratio is available if the total comes back in the gray zone — it saves a second draw. Keep the report: a baseline value and the trend over years are the most useful things a PSA can give you.