Prostate specific antigen, or PSA, is a protein produced by the cells of the prostate gland in men. A small proportion of the PSA in the gland enters the bloodstream, and can be measured in the laboratory. PSA blood levels are most often used as a screening test for prostate cancer, but other prostate disorders can also cause elevations.
Measuring PSA
A blood sample for PSA is collected by a phlebotomist, usually from a vein in the arm. The specimen is taken to a laboratory, where the PSA level is determined using an automated instrument. The results will vary slightly among different laboratories, depending on the methods and instruments used. This difference is important if the PSA result is to be compared with results from months or years earlier.
Normal PSA Levels
A normal PSA is considered to be less than 4 ng/mL, according to the book "Quick Compendium of Clinical Pathology." Levels tend to be lowest in young men, patients who have had their prostate gland removed and in those taking the drug finasteride for prostate enlargement. A normal PSA result makes the presence of prostate cancer much less likely, but does not guarantee the absence of cancer.
Elevated Levels of PSA
The most important cause of an elevated PSA level is prostate cancer. According to the Johns Hopkins Prostate Bulletin, PSA levels between 4 and 10 ng/mL indicate a 20 percent to 30 percent risk of cancer being present. Levels between 10 and 20 ng/mL carry a 50 percent to 75 percent risk, and those above 20 ng/mL have a 90 percent risk of prostate cancer. Noncancerous causes of an elevated PSA include increasing age, benign prostatic hyperplasia, prostate infection and recent prostate biopsy.
Different Uses for PSA
Patients already diagnosed with prostate cancer might also have their PSA levels measured, according to MayoClinic.com. Some men diagnosed with early cancer might choose to delay treatment, and PSA levels can be monitored over time to check for growth of the tumor. Patients who undergo treatment with successful removal of the entire cancer should see their PSA level drop to near 0 ng/mL. A rising PSA level after treatment can indicate recurrence of the cancer.
Recommendations
PSA is a good screening tool for prostate cancer, but by itself can neither prove nor disprove the presence of cancer. As discussed by the National Cancer Institute online at Cancer.gov, not all doctors agree on the best approach to screening with PSA or how to follow up abnormal results. Each individual needs to decide whether to undergo screening only after a discussion with his doctor about the risks and benefits of testing.
References
- "Quick Compendium of Clinical Pathology"; Daniel D. Mais M.D., Editor; 2005
- Johns Hopkins Prostate Bulletin: Understanding PSA
- MayoClinic: PSA Test--Why It's Done
- National Cancer Institute: Prostate-Specific Antigen (PSA) Test
