Prostate Specific Antigen Test


What is the PSA Test?

The PSA test was developed during the late 1970’s by research scientists at Roswell Park Memorial Hospital in Buffalo, New York. The PSA is a blood test that measures the amount of prostate specific antigen (PSA) present in the body. Produced almost exclusively by the prostate gland, PSA is an enzyme, typically present in only minute quantities, secreted into the bloodstream from blood vessels inside the prostate.

PSA secretions originate from cells in the lining of the prostate gland. When prostate cancer is present, additional PSA is usually produced. This extra PSA can be detected and measured in the blood through a simple laboratory test, which can be ordered by any primary-care physician. Test results are usually available in 1-3 days or even longer depending on the assay used.

Because cancerous cells readily leak PSA into the surrounding body tissue, an elevated PSA is a possible indicator of the presence of prostate cancer. However, other conditions can also cause an elevated PSA. The most common is the enlargement of the prostate gland that occurs with BPH. Infections and traumas such as a biopsy or even an overly vigorous digital rectal exam can sometimes increase PSA levels. Ejaculation (orgasm) can elevate PSA for as long as 48 hours.

As a diagnostic tool, the PSA test has its limitations, and is usually combined with the DRE Some men with seemingly normal PSA values turn out to have prostate cancer that my be detected with the DRE or another diagnostic test. A more aggressive cancer may be associated with a palpable tumor (a least a billion cells) found by DRE and normal or even low PSA.

How are PSA Results Reported?

Standard PSA test results are reported in nanograms per millileter (ng/ml), with a normal range of approximately 0-4 ng/ml. For the sake of simplicity, the units of measure will not be included in the remainder of this discussion. The normal range of PSA values must be adjusted slightly to account for differences in age and race (see the chart above). As men get older, the normal PSA range slowly increases. This normal range is generally lower for Caucasians than for Asians and Afro-Americans.

Regardless of age and race factors, PSA levels greater than 10 are most often an accurate indicator of cancer. As many as 80 percent of men with this high a PSA reading (and positive digital rectal exam) have been shown to have prostate cancer. Approximately 25 percent of those patients with a PSA between 4 and 10 turn out to have cancer, as confirmed by standard prostate biopsies. The accuracy of the PSA test is significantly improved when it is combined with the digital rectal exam. The PSA can detect twice as many cancers as the DRE alone; however, the DRE spots some cancers that may be missed by the PSA.

It should be stressed that the PSA test is not conclusive by itself in diagnosing prostate cancer. No treatment decision should ever be made on the basis of the PSA value by itself; however, an elevated PSA reading may suggest the need for further laboratory tests. A biopsy of the prostate gland is always necessary to confirm the presence of cancer.

Because the PSA test in not completely reliable as far as its predictive value, a patient with a high PSA level may not necessarily have cancer and a patient with a very low PSA may not be cancer free. In fact, high grade, more aggressive cancers can lose their resemblance to prostate cells altogether and may not even produce PSA. The PSA provides only a statistical approximation, and there are often exceptions. PSA results are discussed in terms of probabilities the likelihood of prostate cancer being present, and the likelihood that it may have spread beyond the prostate gland.


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