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What is the Prostate Specific Antigen?

The prostate specific antigen (PSA) is a substance produced by certain cells in the prostate gland. PSA is a protein and is a part of semen that causes it to liquefy (keeps the semen watery). Most of the PSA produced by the prostate gland is carried out of the body in semen, but a very small amount escapes into the blood stream.

How is PSA measured?

PSA is tested in the blood. Since the amount of PSA in the blood is very low, detection of it requires a very sensitive type of technology (monoclonal antibody technique).

The PSA protein can exist in the blood by itself, or it can join with other substances in the blood. When it is by itself, it is known as free PSA. When it is joined with other substances, it is known as bound or complexed PSA. Total PSA is the sum of free and bound forms. This is what is measured as the standard PSA test.

What are normal results for the PSA test?

The most frequent value used as the highest normal level is 4 ng/mL (nanograms per milliliter). However, since the prostate gland generally increases in size and produces more PSA with increasing age, it is normal to have lower levels in young men and higher levels in older men. Age-specific PSA levels are as follows (age group, upper normal): (40 - 49, 2.5), (50 - 59, 3.5), (60 - 69, 4.5), (70 - 79, 6.5). The use of age-specific PSA ranges for the detection of prostate cancer is controversial. Not all studies have agreed that this is better than simply using a level of 4 ng/mL as the highest normal value.

How is PSA used to detect prostate cancer?

The PSA test is used in two distinctly different ways with respect to detecting prostate cancer. It can be used in men who are not known to have the disease (screening or diagnostic test) and those who are known to have the disease (tumor marker or monitoring test).

As a screening or diagnostic test, an abnormal result will usually require additional testing. Levels above 4 ng/mL but less than 10 ng/mL are suspicious. However, most men who have this level of abnormality will actually not have prostate cancer. As levels increase above 10 ng/mL, the probability increases dramatically.

As a tumor marker or monitoring test, an abnormal result indicates recurrence of prostate cancer following initial therapy. For example, if the prostate gland is surgically removed (prostatectomy), and all of the cancer is contained within the gland, then the PSA should drop to zero. If on subsequent testing the PSA test is positive and shows increasing levels, then not all of the cancer was successfully removed and it has spread.