Analytical Performance of Free PSA Immunoassays: Results from an Interlaboratory Survey,

Abstract

Prostate-specific antigen (PSA) is a glycoprotein that is secreted by the prostate into the seminal fluid. Low concentrations of the protein are normally released into blood, but in prostate cancer (CAP) as well as in a high proportion of subjects with benign prostatic hyperplasia (BPH), serum PSA concentrations frequently increase above normal values (4 μg/L). Therefore, immunoassays measuring serum PSA concentration are routinely carried out to diagnose prostate diseases and to monitor progress of the disease and relapse of CAP after removal of the prostate (1)(2)(3). A few years ago, PSA was reported to be present in serum in three different forms. The predominant molecular form is complexed to α1-antichymotrypsin, whereas a minor fraction circulates in a free noncomplexed form. These two forms are both measured by PSA assay. Only a very small proportion of PSA circulates bound to α2-macroglobulin; this third form, however, is a nonimmunoreactive complex. The free, noncomplexed form of PSA is reported to constitute a minute proportion of the serum PSA in patients with CAP, but to be significantly greater in subjects affected by BPH. On the basis of this observation the simultaneous measurement of total PSA (tPSA) and free PSA (fPSA) has been suggested. The computed ratio fPSA/tPSA is considered a useful tool to better discriminate between BPH subjects and CAP patients and therefore to improve the early diagnosis of CAP (4). …