What is the difference between free PSA and total PSA?


Quick Answer

Free PSA, or prostate-specific antigen, is PSA circulating unbound and by itself within a man's body. Total PSA is the sum of unbound PSA and PSA bound to other substances circulating within a man's body, according to MedicineNet.com.

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Full Answer

PSA is a protease — an enzyme that breaks down proteins. The prostate secretes PSA to liquefy semen following ejaculation. Most PSA exits a man's body via ejaculation, but some manages to enter the bloodstream. PSA travels through the bloodstream alone or bound with proteins. This is sometimes referred to as complexed PSA, explains MedicineNet.com.

Prostate cancer can elevate PSA. Doctors use a ratio of free to bound PSA as an indicator of prostate cancer. Men with a level of free PSA below 10 percent of a total PSA between 4 and 10 nanograms per millileter of blood are usually encouraged to have a biopsy, reports Harvard Medical School.

Old age, an enlarged prostate or an infected prostate can also raise PSA levels, so high levels aren't necessarily indicative of a cancerous prostate. Men with "normal" PSA levels might be at risk for prostate cancer as well, and Harvard Medical Center cites documented cases of this scenario. Because of this, some doctors order both a PSA test and a biopsy to pinpoint a diagnosis.

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