Test for Free PSA

Dec 27 2000 15:43:31
Kathleen Purcell
drkoop.com Health Correspondent

If you are diagnosed with a prostate problem, your doctor may want to measure your prostate specific antigen, or PSA, levels. PSA is a protein enzyme that converts a man's ejaculate from a gel to a liquid so the sperm can swim out and fertilize the egg.

In a healthy man, great quantities of PSA are seen in semen, and very small amounts are found in the bloodstream. When PSA is found in the blood in sufficient concentrations, doctors suspect prostate cancer.

To look for cancer, doctors traditionally perform a biopsy on the prostate, but there is a test you can request first that may help you decide whether to undergo the biopsy. That test is called the free PSA test.

William Catalona, M.D., professor of urology at Washington University School of Medicine in St. Louis, Mo., first introduced PSA as a screening test for prostate cancer in 1991 in an article published in the New England Journal of Medicine in April of that year.

Catalona explains that because PSA does not belong in the bloodstream, the only time it is found there is when the prostate is diseased. As soon as PSA gets into the blood it is bound by various blood proteins to neutralize its enzyme activity. This form of PSA is called bound PSA.

When PSA becomes damaged, it no longer binds to proteins. Instead, it floats freely through the bloodstream and is called free PSA.

"Normally the concentration of PSA in the blood should be less than 4 nanograms per milliliter of blood," Catalona said. "When the total PSA is elevated, we suspect cancer."

If a blood test shows that the total PSA level is high, a free PSA test can be given to test the percentage of bound vs. the percentage of free PSA. "For reasons we do not understand, most men who do not have cancer will have about 75 percent bound PSA and 25 percent free. But if cancer is present, we find something like 90 percent bound and only 10 percent free PSA," Catalona said.

According to Catalona, the test is most useful in the diagnostic gray zone: when a man's total PSA is higher than 2.5 but lower than 10 nanograms per milliliter of blood. That's because when total PSA reaches 10 and above, there is a greater than 50-50 chance cancer is present, and when total PSA is lower than 2.5, there is very little chance of cancer. But between 2.5 and 10, a surgeon must make a diagnostic judgement call, and additional information is welcome. A free PSA test can give a surgeon that additional information.

"Let's say you have a man in his 70s and he has a slightly elevated total PSA but his free PSA is also high," Catalona explained. "That man might forgo a biopsy because his chances of having cancer are only 1 in 12. Whereas if you had a patient in his 50s with the same total PSA but a free PSA of only five, that man has a 50-50 chance of having cancer and he might opt to have a biopsy."

"The numbers can get confusing," Catalona added. "When you're measuring total PSA, the higher the concentration, the worse your prognosis. But when you're measuring free PSA, the lower the number the worse it is."