Prostate-specific antigen (PSA) takes two major forms in the blood. One is attached, or bound, to blood proteins and the other circulates freely. The percent-free PSA test indicates how much PSA circulates free compared to the total PSA level. The percentage of free PSA is lower in men who have prostate cancer than in men who do not. Studies show that if your PSA results are in the borderline range (4 to 10), a low percent-free PSA (less than 10%) means that the likelihood of having prostate cancer is about 50% and that you should probably have a biopsy.
Some doctors recommend biopsies for men whose percent-free PSA is 20 or less.