Free PSA

Prostate Specific antigen (PSA) is a serine protease
with chymotrypsin-like activity. PSA is a single chain
glycoprotein with a molecular weight of 28.4 kDA.
PSA derives its name from the observation that it is a
normal antigen of the prostrate and is not found in
any other normal or malignant tissue. PSA is released
from the normal prostate and appears at low serum
concentrations in healthy men. Studies with reverse
transcription-PCR have shown that PSA also is
expressed at a low concentration in peripheral blood
cells and other tissues. High serum concentrations
can be detected in patients with advanced prostate
cancer (PCA). Therefore PSA is applied as a tumor
marker for the clinical management of PCA. However,
increased PSA concentrations in serum also occur in
patients with benign prostate hyperplasia (BPH).
Hence the goal in clinical laboratory is to discriminate
clearly between BPH and PCA to spare the patient
invasive diagnostic procedures, such as a prostate
biopsy.
In human serum PSA occurs in two forms: free PSA
(f-PSA) and complexed PSA. The major form is a
complex of PSA and α1-antichymotrypsin (ACT). The
fraction of f-PSA was shown to be substantially
smaller in patients with untreated PCA than in
patients with BPH. Therefore combined
measurements of f-PSA and total PSA (t-PSA) may
lead to a better discrimination between BPH and PCA
Some recent studies have already shown that the fPSA/t-PSA
ratio is helpful in the differential diagnosis
of BPH and PCA.
PSA is found in benign, malignant and metastatic
prostrate cancer. Since prostate cancer is the
second most prevalent form of male malignancy, the
detection of elevated PSA levels plays an important
role in the early diagnosis. Due to increased
sensitivity, serum PSA levels have been found to be
more useful than prostatic acid phosphatase (PAP) in
the diagnosis and management of patients.