What is a PSA test?

This blood test measures the level of a protein called prostate-specific antigen (PSA), which is manufactured by the prostate. Doctors use the test to detect prostate cancer, but it does not provide a definite diagnosis. That’s because an above-normal PSA level — more than 4 nanograms per milliliter (ng/ml) of blood — doesn’t always indicate the presence of cancer. Likewise, levels of 4 ng/ml or less don’t always mean the prostate is cancer-free. For this reason, some experts adjust for a man’s age or consider a normal level to be 2.5 ng/ml. As doctors learn more about how to use the PSA test, they are less likely to use it as a simple, one-time indicator and, instead, to observe how a man’s PSA level changes over time and with age.

The PSA test can provide clues to other conditions besides cancer. Indeed, most men who have mildly elevated PSA levels don’t have cancer. Elevated PSA levels can accompany the noncancerous conditions known as benign prostatic hyperplasia and prostatitis. Complicating things further, an elevated PSA may simply mean that a man’s prostate naturally releases more of the protein into the blood. Moreover, some studies suggest that ejaculation can elevate PSA levels for up to 48 hours. In addition to using the PSA test to identify problems, doctors also use it for follow-up after prostate cancer treatment and to predict which patients with BPH are likely to develop troublesome symptoms.

The PSA test has grown popular since it became widely available in the late 1980s. But experts disagree about whether PSA testing should be routine for men over age 50. The death rate from prostate cancer in the United States has dropped, and some experts believe that the explanation is widespread PSA testing. Others, however, attribute the decline to other factors, such as the decreasing amount of fat in the American diet. They point out that prostate cancer mortality rates have also declined in England and Wales, where PSA screening is rare.

Because an elevated PSA can’t differentiate cancer from BPH or prostatitis, it can cause needless worry — and may lead to costly and invasive procedures, such as biopsy, in order to determine if cancer is present. Conversely, the PSA doesn’t detect all cancers, so a normal PSA level may offer a false sense of security. Many men with cancer confined to the prostate have normal PSA values. Even advocates of PSA testing doubt its value in men with less than a 10-year life expectancy (ages 75 or older, for men in average health) because these men are more likely to die of something else first.

The PSA can, however, help detect prostate cancers that are too small to feel during a DRE. Indeed, the test has been credited with finding many more cancers in early stages than had been found in the past. But it’s not yet known if earlier detection actually improves people’s chances of survival, because prostate cancer’s slow growth means researchers need very long follow-up periods to see results.

PSA velocity. Doctors are increasingly using 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 BPH. A 2004 study in The New England Journal of Medicine showed that men with prostate cancer who have rapid increases in PSA were more likely to die from the cancer than those with slower-rising levels.

Free PSA. Doctors sometimes look for the level of “free” PSA. PSA protein 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. Still, even men with a low percentage of free PSA have only about a 10% chance of having prostate cancer. Most physicians find that PSA velocity is better than free PSA at predicting the need for biopsy. Therefore, if the patient has had several PSA tests over a period of years, doctors are less likely to test for free PSA.

The American Cancer Society advises doctors to offer their patients annual PSA tests beginning at age 50, or age 45 for men at high risk. The American Urological Association likewise endorses this view.

On the other hand, the American College of Physicians and the American College of Preventive Medicine advise against routine screening. But they do recommend that physicians run through the potential benefits and drawbacks of the PSA test with their patients to help them decide whether to be screened (see “What you should know about PSA screening”).

Originally published March 2009; last reviewed March 21, 2011.