Prostate specific antigen (PSA) is a protein produced by prostate cells. Mostly is it carried out of the body in semen, but a very small amount escapes into the blood stream.
PSA test measures the level of PSA in the blood in men who are not known to have prostate cancer (screening or diagnostic test) and those who are known to have the prostate cancer (tumor marker or monitoring test).
The highest normal value used most frequently is 4 ng/mL (nanograms per milliliter), but the prostate gland generally increases in size and produces more PSA with increasing age so the highest normal levels also change with age as follows:
- age 40 – 49 2.5
- age 50 – 59 3.5
- age 60 – 69 4.5
- age 70 – 79 6.5
Some studies show that is better to use 4ng/mL, as the highest normal value and some studies that are better to use values by age.
When the test is used as a screening or diagnostic test, an abnormal result (levels above 4 ng/mL but less than 10 ng/mL) will usually require additional testing, but most men who have this level of abnormality will actually not have prostate cancer if the level does not increase above 10 ng/mL, because this increases the probability dramatically.
When the test is used as a tumor marker or monitoring test, an abnormal result indicates recurrence of prostate cancer following initial therapy. As an example, if the prostate gland is surgically removed (prostatectomy) with cancer, then the PSA should drop to zero. If the PSA test is positive and shows increasing levels later, then not all of the cancer was successfully removed and it has spread.
PSA can be also elevated in men who have an infection or inflammation of the prostate or benign prostatic hyperplasia (BPH) or hypertrophy (enlargement). These conditions are due to an increase in the size of the prostate gland that typically occurs with aging.
The PSA protein released into the blood becomes attached to other blood proteins and if it is not attached, it is known as free PSA. Free PSA can be decreased in men who have prostate cancer compared to those with benign conditions and the exact level depends on upon which tests the laboratory uses, but generally less than 10% free PSA is suggestive of cancer so this is helpful when the usual PSA test is between 4 and 10 ng/mL.
The limitations of the PSA test are that many early cancers cannot produce enough PSA to cause a significantly abnormal blood level, and also few percent of prostate cancers do not produce detectable increases in the blood PSA, even with advanced disease. It is important not to rely only on a blood PSA test.