PSA screening is a diagnostic test for men that checks blood levels of prostate-specific antigen, or PSA. Prostate-specific antigen is present in low levels in all men. Abnormally high levels of the antigen indicate that prostate cancer might be present. PSA screening is carried out via a simple blood test, the results of which are obtained through laboratory analysis.
Prostate-specific antigen is produced in the prostate gland. The fluid produced by this gland makes up about one-third of the volume of semen. The addition of PSA to semen makes the fluid more liquid and improves the motility of sperm. PSA also is thought to be important in dissolving the cervical mucus cap and allowing sperm access to the uterus.
Prostate cancer tends to cause the prostate gland to produce more PSA. This tendency allows PSA screening to be used to screen for diseases of the prostate. The test is not foolproof, however, because there are other reasons why PSA levels might be elevated. Infection of the bladder, ureters and urethra and infection or inflammation of the prostate can all cause a detectable increase in PSA levels.
The prostate gland enlarges with age, regardless of whether it is affected by illness or disease. When the prostate is enlarged, it produces larger amounts of PSA. Sometimes, a man might have high levels of PSA because of age and not because of disease. It also is possible for a man to have prostate cancer and also have a normal PSA test result.
Even though PSA screening is not a highly accurate predictor of prostate cancer, it still is a useful diagnostic tool. Healthy baseline PSA levels based on age have been determined, which means that test results for older men generally can rule out age-related PSA increases. The fact that both false positive and false negative test results are possible, however, means that the PSA blood test is always used in conjunction with other tests for screening and diagnosing prostate cancer.
Of those men who test positive for abnormal PSA levels, about one-third will be diagnosed with prostate cancer. In men who test positive, PSA screening is followed up by diagnostic tests such as rectal examination to improve the accuracy of the diagnosis. These two tests do not comprise the entire diagnostic process, however, because only 50 percent of men with an abnormal PSA level and abnormal rectal exam are diagnosed with prostate cancer. Screening tests such as the PSA test are carried out not to diagnose cancer, but to screen for risk factors. The only way to accurately diagnose prostate cancer is by taking a biopsy of the gland and testing the tissue for cancer cells.