... tests PSA what is the difference between total PSA and free PSA? What is the normal range? In addition, how can it be that if there is ANY prostate specific antigen in the blood that there is ... protein produced by the cells of the prostate gland. The most frequently used PSA - prostate-specific antigen test - is the total PSA, which measures the sum of the free PSA and the cPSA (complex ... method. Differences between test and comparison methods were estimated at 2.5, 4.0, and 10.0 ng/mL (2.5, 4.0, and 10.0 µg/L) for tPSA and 15%, 20%, and 25% for percentage of fPSA. Relative differences...