The PSA test or prostate specific antigen test test tells your doctor the level of prostate specific antigen -- an enzyme made by the prostate -- in your blood, just as a cholesterol test tells the levels of cholesterol in your blood.

There is no one PSA reading that is considered "normal." The reading varies from man to man. But a value around 4 or less is considered to be in the normal range. However, since PSA scores tend to increase with age, the chart below indicates what is viewed as a normal value for men ranging from 40 to 80 years of age.

A PSA level of up to 10 ng/mL is considered slightly elevated; values between 4 and 10 ng/mL are considered the most ambiguous.

Levels between 10 and 20 ng/mL are considered moderately elevated, with a significantly increased risk of prostate cancer. Anything above that is considered highly elevated. Above 20 ng/mL, the risk of cancer is rated as high as 90 percent.

The PSA test is a far from definitive screening tool. An elevated PSA can be a sign of cancer but your PSA level can also be raised in benign prostate conditions. Urological difficulties, such as prostatitis or urinary retention, and invasive procedures, such as a prostate biopsy, may raise PSA levels. For men over 50, ejaculation within the previous two days may also raise PSA levels.

Conversely, prostate cancer is quite often found in men with low PSA values.

But whatever its deficiences for screening, once cancer has been diagnosed, the PSA level is viewed as a useful tool for assessing its pathological stage. The higher the level of PSA, the more likely the cancer is to spread quickly. If PSA levels are less than 20 mg/mL, it is likely the cancer has not spread. A PSA level over 40 mg/mL, on the other hand, is viewed as a strong indication that the chance has metastasized. Some men have PSA levels over more than 100 mg/mL when they are diagnosed.

The PSA level is also one component -- together with Gleason score and estimated clinical stage -- of the "Partin tables," designed to help you and your doctor determine the best course of treatment.

Following initial treatment of prostate cancer, your PSA level will continue to be monitored for signs your prostate cancer is recurring or progressing. If your PSA is stable, it is a sign that cancer is not on the move. A rising PSA level often indicates recurrence.