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Complexed PSA discussed in relation to
standard PSA test (December 2002)
Maybe PC testing using the new "Complexed
PSA" test offers a small step in a positive direction. A full supplement
to the journal UROLOGY, October 2002, was devoted to a discussion of
complexed PSA (cPSA). The editor for the supplement was Dr. Michael
Brawer, Director, Northwest Prostate Institute, Seattle.
Basic biology first: Total PSA (tPSA), the usual value reported,
represents the sum of two main elements: cPSA and free PSA (fPSA), which
is "uncomplexed". cPSA is coupled ("complexed") with an inactivator,
mainly alpha 1-antichymotrypsin, which restrains the circulating PSA's
enzymatic activity. As the percentage of fPSA relative to tPSA decreases
to less than 20-25%concern for underlying cancer increases. The Bayer
Company has developed a test that uses an antibody that identifies tPSA
and at the same time adds a specific antibody that blocks the measurement
of fPSA. Hence, in a single test the "complexed PSA" is measured. The
basic issue is whether cPSA testing improves the specificity for cancer
detection compared to the standard "PSA". An additional question is
whether cPSA can be a substitute for the fPSA/tPSA ratio, a quotient that
in current testing requires two tests and a calculation, which may
introduce a compounding error. A single test has comparative economic
value. Studies have shown that fPSA begins to deteriorate in storage at 4
degree F over several days. (Dynacare stores serum at -20 degrees F.)
How does the complexed PSA test measure up? Horninger in the supplement
reported the results of biopsies in 215 men with PSA's between 2-4, some
having suspicious DRE's. In the Horninger study 57 of 158 (26.5%) men were
biopsy positive. After refining their data they reported biopsy positivity
in 20.3% when using the PSA range of 2.5-4. When cPSA testing was applied
to the same study group and a cutoff range of cPSA of 2.1 was used,
positive biopsies were found in 34.2%. (cPSA 3.33 is approximately
equivalent to PSA 4,since, as mentioned, cPSA doesn't measure fPSA). If
testing with cPSA can be shown to have greater specificity for finding PC,
unnecessary biopsies can be avoided. Further studies comparing cPSA to PSA
for PC screening are in progress.
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