PSA is a 32 kDa single-chain, glycoprotein serine protease with a chymotrypsin-like specificity produced by the secretory epithelium of the prostate gland. PSA is normally secreted into the seminal fluid and plays a functional role in the cleavage of the seminal vesicle proteins and the liquefaction of the seminal coagulum. Only low levels of PSA are normally present in the blood stream, and increasing serum concentrations indicate prostatic pathology, including benign prostatic hyperplasia and cancer of the prostate. Determination of PSA is now widely used for detection and management of patients with prostatic cancer and considered as the superior serological marker for cancer of the prostate.

PSA has been shown to form stable complexes with different antiproteases and the dominating portion of PSA in patient serum occurs in complex with α1-antichymotrypsin (PSA-ACT). However there are large variations in the relationship between Free PSA and PSA-ACT complex between different individuals. A number of studies have found that the proportion of Free PSA is higher in benign prostatic disease as compared to prostatic cancer. The Free PSA EIA is an assay for specific determination of Free PSA without cross reactivity with PSA-ACT complex.