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 several distinctly different ways with respect to detecting prostate cancer. It can be used in men who are not known to have the disease as a screening test. It can be used to gauge the aggressivity of prostate cancer. And it can be used in men with prostate cancer to monitoring their disease.
- Screening test: An abnormal PSA result usually requires additional testing. Levels above 4 ng/mL but less than 10 ng/mL are considered suspicious. However, most men who have this level of PSA do not have prostate cancer. As the PSA levels increase above 10 ng/mL, the probability of prostate cancer increases dramatically.
- Gauge of aggressivity: Men whose PSA level rises rapidly (more than 2 ng per milliliter) during the year before the diagnosis of prostate cancer have a higher risk of dying from prostate cancer despite undergoing radical prostatectomy (surgery that removes the entire prostate gland plus some surrounding tissue).
- Monitoring test: An abnormal PSA result following therapy may indicate recurrence of the 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 confounding 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).