Definition 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 examination 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 After the needle is withdrawn; the puncture site is covered for a short time to prevent bleeding.
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 doctor can provide specific information on preparation for the test.
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.
Current debate on PSA testing for the detection and monitoring of prostate cancer is based on the performance of total PSA assays. New evidence to date indicates the estimation of free PSA, total PSA and the free/total PSA ratio may reduce the need for prostate biopsy. This may soon transform the current debate.
Author: David T. Moran, MD Reviewer: eknowhow Medical Review Panel Editor: Dr John Hearne Last Updated: 6/06/2005 Contributors Potential conflict of interest information for reviewers available on request
