Prostate Specific Antigen
(PSA)

• What is the Prostate Specific Antigen?
• How is PSA measured?
• What are normal results for the PSA test?
• How is PSA used to detect prostate cancer?
• What other conditions can cause an abnormal PSA level?
• What is the free PSA test?
• What are the limitations of the PSA test?
• PSA Test At A Glance

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.

Next: How is PSA used to detect prostate cancer? »

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Last Editorial Review: 4/24/2002