What is prostate-specific antigen (psa)?

Marc B. Garnick, MD
Hematology & Oncology
A prostate-specific antigen (PSA) test is a blood test that measures the level of a protein called prostate-specific antigen (PSA) made by cells in the prostate.

Rather than subject everyone with an elevated prostate-specific antigen (PSA) level to a biopsy, especially patients with a level between 4 and 10 nanograms per milliliter (ng/mL), some doctors will do additional PSA tests. (Only about 25% of men with a PSA in this range have prostate cancer.) These tests include the following:

PSA velocity. Increasingly, doctors use a measure called PSA velocity or series. This is the rate at which a man's PSA level increases over time. PSA scores tend to rise more rapidly in men with cancer than in those with benign prostatic hyperplasia (BPH).

PSA velocity has undergone a great deal of research and at the current time should be considered a measurement that may provide useful information, but there are many variations in its interpretation, making it less useful for large groups of men.
Free PSA. Doctors sometimes look for the level of "free" PSA. PSA circulates in the blood in two forms, either bound to other proteins or unbound (free). Several studies suggest that men with elevated PSA levels who have a very low percentage of free PSA are more likely to have prostate cancer than a benign prostate condition. The free PSA level doesn't give a definitive answer, but it may be useful when considering whether a biopsy is the appropriate next step. Many doctors recommend biopsies in men with a free PSA level of 10% or less, and advise men to consider a biopsy if the free PSA level is between 10% and 25%.

PSA density. This number is calculated by dividing the PSA level by the size of the prostate in grams, as measured by ultrasound. (Prostate size can't accurately be determined by DRE because only the back of the gland can be felt.) The higher the PSA density, the greater the likelihood of cancer. In general, doctors consider a PSA density under 0.07 indicative of a benign condition; a PSA density greater than 0.15 is suspicious for cancer. If your PSA density falls between those two numbers, its meaning is uncertain.

Prostate-specific antigen (PSA) is a substance that the prostate makes that is added to semen that makes it more fluid to help carry sperm and to aid ejaculation. It is normally present in the blood in only very small amounts because there is a good prostate-blood barrier in the absence of prostate problems. The PSA test measures levels of PSA in the blood. This is the best available marker for prostate cancer because cancer interrupts the barrier between prostate and blood, so levels in the blood increase. This is the earliest indication of cancer. The limitation of the test is that cancer is not the only thing that increases PSA levels. When the prostate gland enlarges with age (benign prostatic hyperplasia, or BPH), more PSA is produced and more gets into the blood. Other conditions, notably inflammations of the prostate, can disrupt the prostate-blood barrier and result in elevated blood PSA levels. Scientists are studying ways to improve the ability of the PSA test to target cancer.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.