Prostate-specific antigen (PSA ) exists as a protein that may or may not be bound to plasma proteins in the bloodstream and may or may not be free.
- A free PSA is free PSA, denoted as fPSA.
- PSA that is bound to plasma proteins is called complex PSA and is referred to as cPSA.
- PSA in the blood is the sum of free PSA and complex PSA, also known as total PSA, and is expressed as tPSA.
Tests typically measure both tPSA and fPSA, and results usually include tPSA values, fPSA values, and the ratio of fPSA to tPSA (fPSA/tPSA).
Most studies have shown that fPSA is effective in increasing the detection rate of prostate cancer with tPSA levels between 4 and 10 ng/ml. When serum tPSA is between 4 and 10 ng/ml, fPSA levels are negatively correlated with the incidence of prostate cancer, meaning that the smaller the fPSA/tPSA ratio, the higher the likelihood of prostate cancer in that patient.
Domestically, an fPSA/tPSA greater than 0.16 is recommended as a normal reference value.