Antigen, prostate specific (PSA)
A test used to screen for cancer of the prostate and to monitor treatment. PSA is a protein produced by the prostate gland. Although most PSA is carried out of the body in semen, a very small amount escapes into the blood stream. The PSA test is done on blood. Since the amount of PSA in blood is normally minute, the PSA test requires a very sensitive method based on monoclonal antibody technology. PSA in blood can be by itself as free PSA or it can join with other substances in the blood as bound PSA. Total PSA is the sum of free and bound forms. This is what is measured as the standard PSA test. The PSA value used most frequently as the highest normal level is 4 ng/mL (nanograms per milliliter). However, since the prostate gland generally increases in size and produces more PSA with increasing age, it is normal to have lower levels in young men and higher levels in older men. Age-specific PSA levels are as follows (age group, upper normal): (40 - 49, 2.5), (50 - 59, 3.5), (60 - 69, 4.5), (70 - 79, 6.5). The use of age-specific PSA ranges for the detection of prostate cancer is controversial. Not all studies have agreed that this is better than simply using a level of 4 ng/mL as the highest normal value. The PSA test is used in two distinctly different ways with respect to prostate cancer. It can be used in men who are not known to have the disease as a screening test and it can be used in men known to have the disease as a tumor marker or monitoring test: {{}}Screening test: An abnormal result usually requires additional testing. Levels above 4 ng/mL but less than 10 ng/mL are suspicious. However, most men who have this level of abnormality will actually not have prostate cancer. As levels increase above 10 ng/mL, the probability of prostate cancer increases dramatically. Monitoring test: An abnormal result following therapy indicates recurrence of prostate cancer. PSA is not specific to prostate cancer. Other diseases can cause an elevated PSA. The most frequent is benign prostatic hypertrophy (BPH), an increase in the size of the prostate that typically occurs with aging. Infection of the prostate gland (prostatitis) is another relatively common cause of an elevated PSA. Other conditions that can increase PSA include ischemia or infarction, urethral instrumentation, and urinary retention, and prostate biopsy. The PSA test has other limitations. A small proportion of prostate cancers do not produce a detectable increase in blood PSA, even with advanced disease. Many early cancers will also not produce enough PSA to cause a significantly abnormal blood level. It is therefore important not to rely only on blood PSA testing. The most useful additional test is a physical prostate exam known as the digital rectal exam (DRE).

Medical dictionary. 2011.

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