Prostate Specific Antigen (PSA) test for detecting prostate cancer

Physicians usually order tests for blood PSA levels when assessing a patient who has a suspicious nodule or lump on his prostate. Increasingly, the PSA test is being used in patients whose prostate gland feels normal during the rectal examination but who have requested an evaluation because of a concern or predisposition for prostate cancer, e.g., a close family member has the disease. However, the PSA test by itself does not diagnose prostate cancer. Rather, it provides a clue that cancer may or may not be present.

Prostate specific antigen is a substance that is produced only by prostate tissue cells, but a high level is not necessarily indicative of cancer. In other words, it can be high in both cancerous and noncancerous situations. For example, 20% of men with benign prostatic hyperplasia (BPH) have a higher than normal level of blood PSA, and as many as 70% of all men with an above-normal PSA reading of between 4.0 and 10.0 do not have cancer. On the other hand, up to 20% of men who are diagnosed with prostate cancer have PSA levels in the normal range, below 4.0.

PSA levels may be increased by a number of non-malignant conditions other than BPH. For example, a prostate infection may cause quite a high reading. Treatment of the infection with an antibiotic may bring the PSA level back to normal, but if cancer is still suspected then further diagnostic tests should be done. Also, pressure or trauma to the perineum or buttocks from, for example, bicycle riding, can cause quite significant elevations, as can stress or pressure on the prostate itself from a digital rectal exam, biopsy, or vigorous sexual activity.

The actual function of PSA is related to fertility in that it prevents the seminal fluid from coagulating and helps maintain the health of the sperm cells after ejaculation into the vagina. Every male has a certain amount of PSA circulating in his bloodstream. The "normal value" is generally considered to be under 4.0 ng/mL (nanograms per milliliter). (This value may vary slightly depending on the methodology used to measure it.)

Scientists are refining the way that they interpret PSA results to try to distinguish cancer more accurately from non-malignant situations. For example, an age-related scale has been suggested to account for older men naturally having larger, "leakier" prostates that allow more PSA to flow into the bloodstream. Thus, the upper limit of normal for a 45-year-old may be 2.5, while a 75-year-old man's normal PSA level may be 6.5.

Two other aspects also need to be considered when trying to interpret the significance of the PSA level. The first has to do with normal individual variations in the size of the prostate gland. Thus, a man with a naturally large prostate gland may have a high PSA level when considered on its own, but a normal ratio of PSA level compared to prostate size (called the PSA density).

The second aspect that needs to be taken into account is the rate of change of the PSA level over time (known as PSA velocity). For example, a man may have a normal PSA level of 1.0 one year, and then a higher, but still normal level of 3.5 the next. Although both readings are in the normal range, the rapid increase is significant and would indicate the need to rule out a cancer, since benign tissue does not exhibit such a rapid increase within such a short period of time.

In today's world of medicine, the PSA test stands out as a superb "marker" of abnormalities in the prostate gland. When the PSA level is supplemented by the other considerations mentioned here, the test will help detect those prostate cancers that are truly significant, hopefully before they have had a chance to grow too extensively.

Excerpt from the Intelligent Patient Guide to Prostate Cancer by S. Larry Goldenberg, MD 
in association with the MediResource Clinical Team