If I have a low PSA level, could I still have prostate cancer?

A low PSA (prostate specific antigen) level does not rule out the possibility that a man has prostate cancer, according to a new National Cancer Institute study. This is the first large study to look at how often prostate cancer is diagnosed among men with low PSA levels.

PSA is a substance produced by the prostate. It can be measured by a simple blood test. A high PSA level may be caused by prostate cancer or one of several less serious conditions. A biopsy (surgical removal of tissue to check for cancer) and other tests are often recommended if the patient has a PSA level of 4 nanograms per milliliter or above. Levels under 4 are generally considered normal.

In the NCI study, almost 3,000 men age 55 and older with normal PSA levels underwent prostate biopsies. Fifteen percent were found to have prostate cancer. Most of the cancers were slow growing and likely to remain harmless. Many men over age 50 have early, undiagnosed prostate cancer, according to autopsy reports. Doctors agree that most of these cancers will remain harmless, although some may become aggressive.

About 2.3 percent of the cancers found in the study were aggressive. This means they are more likely to grow quickly and spread to other parts of the body.

It is unclear how doctors will respond to the new findings. The PSA test is commonly used in the United States to find prostate cancer early. However, some scientists are concerned that its widespread use may lead to "over-diagnosing" the disease and unnecessary biopsies and treatment for men with slow-growing cancers.

More research on prostate cancer screening is needed, and the NCI has several studies underway. Meanwhile, men with questions should talk with their doctor.

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