Our laboratory performs the following tumor marker assays: calcitonin on the Nichols Advantage, gastrin on the Immulite2000, CA 125 and cancer antigen 15-3 (CA 15-3) on the Vitros ECi, and a-fetoprotein (AFP), hCG, total PSA, and free PSA
on the Beckman Access.
Prostate specific antigen (PSA) and free PSA
(fPSA) assays are invaluable clinical tools in the healthcare arena.
Approximately 70-90% of tPSA is bound to [[alpha].sub.1]-antichymotrypsin, [[alpha].sub.1]-protease inhibitor, or inter-[alpha]-trypsin inhibitor (8-10); the remaining 10-30% of tPSA is not bound to serum proteins and is called free PSA
The Elecsys Free PSA
(fPSA) immunoassay, with a sensitivity of 0.01 ng/mL, should be used with the tPSA test to develop a ratio of fPSA to tPSA, which is useful in distinguishing malignant from benign prostate conditions in men with an elevated tPSA, but normal DRE.
The free PSA
assay was identical to the total PSA measurement except that a free-PSA-specific MAb (5A10) was used as the tracer (49, 50).
A study in the August issue of Urology(2000;56:255-260) reports that the free PSA
test (iPSA) is the most effective follow-up test for those who fall into the 'gray area" of PSA results, which is between 4 ng/mL and 10 ng/mL with digital exams showing no sign of prostate cancer.
PSA circulates in human serum in two main forms, either free ("unbound") or complexed ("bound") (2); complexed PSA refers to the fraction of total PSA (tPSA) that is bound to any of the proteinase inhibitors, whereas free PSA
(fPSA) is the fraction not bound to any protein.
European researchers tracked prostate cancer deaths in Tyrol, Austria, where 65,000 men ages 45 and 75 were given free PSA
screenings starting in 1993.
The transition and peripheral zone tissues from the same prostate also have two forms of free PSA
, called "benign" PSA (BPSA) and proPSA (18,19).