Prostate specific antigen (PSA) are produced by prostate cells (the tiny basic functioning units of the prostate). The PSA then circulates in the bloodstream. While in the blood, PSA can either be attached to proteins that are present in the blood or it can be unattached (“free”). It is normal to have both attached and "free" PSA and neither of these forms of PSA are considered abnormal.
The "percent-free" PSA can be calculated by dividing the amount of “free” PSA by the total PSA.
Studies have found that, in general, men with prostate cancer have lower levels of “free” PSA when compared to men without prostate cancer.
Normal and Abnormal Percent-Free PSA
Typically, men have a percent-free PSA above 25%. Between 10% and 25% is considered an intermediate range and below 10% is considered to be low.
However, a low percent-free PSA, even coupled with a high absolute PSA level (the raw number that you were told is your PSA number), does not necessarily mean you have prostate cancer. Only a biopsy can definitively diagnose prostate cancer.
Does My Percent-Free PSA Really Mean Anything?
The most common situation in which the percent-free PSA is used to make a decision about how to test for or treat prostate cancer is when the absolute PSA level is at an intermediate level.
In this situation, the physician may recommend a biopsy to a man who has a low percent-free PSA (and therefore a relatively higher risk of cancer) and recommend against a biopsy in a man who has a high percent-free PSA (above 25%).
Percent-Free PSA – A Relatively New Concept
In the past, physicians relied almost exclusively on the absolute PSA level to decide about how likely you were to actually have prostate cancer.
Quickly, however, it became apparent that using just the absolute PSA level failed in some important ways:
For one, some men with normal or even low absolute PSA levels have been found to have prostate cancer.
Second, many men with high absolute PSA levels do not have prostate cancer. Instead, they have a much less dangerous condition such as benign prostatic hyperplasia (BPH).
Basically, the absolute PSA level does not always tell the whole story.
Doctors have now started using alternative PSA measurements such as the PSA density and the percent-free PSA to make better decisions about a man's risk for prostate cancer.
Saraiya M, Kottiri BJ, Leadbetter S., et al. Total and percent free prostate-specific antigen levels among U.S. men, 2001-2002.
Uemura H, Nakamura M, Hasumi H., et al. Effectiveness of percent free prostate specific antigen as a predictor of prostate cancer detection on repeat biopsy. Int J Urol. 2004 Jul;11(7):494-500.
Walz J, Haese A, Scattoni A, et al. Percent Free Prostate-Specific Antigen (PSA) Is an Accurate Predictor of Prostate Cancer Risk in Men With Serum PSA 2.5 ng/mL and Lower. Cancer. 2008 Nov 15;113 (10):2695-2703.