Overview & Description

Prostate-specific antigen (PSA) is a protein. It circulates in the blood and usually binds to other larger proteins. The amount of PSA that is not attached to other proteins is called free PSA. Attached protein is called bound PSA.

The standard PSA test measures the total amount of PSA, both free and bound. Some researchers think that men with prostate cancer have more bound PSA than free PSA compared with men who do not have prostate cancer.

Who is a candidate for the test?

Men who have a high standard PSA level, a normal rectal exam and normal prostate biopsy results are candidates for this test.

How is the test performed?

In order to measure the amount of free PSA in the blood, a blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or "tourniquet", is wrapped around the upper arm to enlarge the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle, and is collected in a syringe or vial for testing in the laboratory. After the needle is withdrawn; the puncture site is covered for a short time to prevent bleeding.

Preparation & Expectations

What is involved in preparation for the test?

The same preparations used for the standard PSA test are recommended for the free PSA test. A healthcare provider can provide specific information on preparation for the test.

Results and Values

What do the test results mean?

Men who have low free PSA levels may be at risk for developing prostate cancer. These men may benefit from more frequent testing to detect prostate cancer. These tests include rectal examinations or prostate biopsies.

The best way to determine if a man has a low free PSA level is to calculate the percentage of free PSA. This is done by dividing the value reported for free PSA by the value reported for the standard or total, which is free plus bound PSA. If the calculated percentage is greater than 23%, the risk of prostate cancer is minimal. If the percentage is less than 10%, the risk of prostate cancer is high or more likely. If the percentage is between 10% and 23%, the risk of prostate cancer is not clear or not well defined.

The free PSA test has not been approved by the US Food and Drug Administration for predicting the risk of prostate cancer. Many laboratories do not offer this test and many insurance companies do not pay for it. Free PSA values between 4 and 10 are reliable and accurate. Values below 4 and above 10 may not be reliable. More information is needed before this test can be used routinely to screen men for prostate cancer, or to monitor the progress of the disease or its treatment.