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PSA TestingHow Much is Too Much?

For more than 20 years, the PSA test has been widely used as the gold standard for prostate cancer screening. In one of the most dramatic reversals in recent diagnostic science, a recent study at Stanford University School of Medicine and findings by other researchers have now put the utility of the procedure in grave doubt.

Indeed, The PSA era is over in the United States, said Thomas Stamey, MD, professor of urology and lead author of the study published in the October 2004 issue of the Journal of Urology. Our study raises a very serious question of whether a man should even use the PSA test for prostate cancer screening any more, Stamey said. The researchers opinion carries great influence, since it was Stameys work in the late 80s that propelled PSA testing into widespread use (and prompted the corresponding practice of routinely removing prostates in response to high PSA levels).

As time has passed, however, Stamey has concluded that PSA testing is unable to predict the extent or severity of prostate cancer but, instead, provides a measure of benign prostatic hyperplasia.

Mark Twain once famously quipped that, The report of my death was an exaggeration. The death knell of PSA may also have sounded too soon, since the assay is likely to continue to provide useful data, albeit in far less universal applications. Stamey and his colleagues identified two important and continuing uses for PSA testing in their recent paper. They noted that PSA levels quite accurately correlate with prostate size and that the test thus serves as a direct measure of benign prostatic hyperplasia. Stamey noted that the assay is also helpful for monitoring patients following prostate removal because it can indicate residual prostate cancer that has metastasized.

As with many disease screens, PSA tests now range from assay reagents used in multi-function clinical laboratory analyzer systems to rapidly performed, self-contained kits suitable for point of care use, to mail-in specimen collection kits available for clinical or home screenings.

Bio-Quant, Inc., San Diego, Calif., manufactures the OneStep PSA InstaTest designed to detect PSA concentration in whole blood or serum in concentrations as low as four ng/ml within 15 minutes. The result is read visually by observing symbols that develop during the test in a result window. The company says that comparative studies have demonstrated that relative sensitivity of InstaTest PSA is 100 percent, and its relative specificity is 98.5 percent.

Beckman Coulter, Inc., Fullerton, Calif., produces the Hybritech PSA Test and, as a confirmatory follow-up, the Hybritech free PSA Test. The Hybritech PSA test was the first assay approved by FDA for the detection of prostate cancer when used in combination with digital rectal examination (DRE) in men aged 50 years or older. The assay provides equimolar PSA measurement based on a monoclonal antibody pair. The company lists analytical sensitivity at <0.008 ng/mL ; measurement range to 150 ng/mL and precision at < 5% CV across the assay range. Time to first result is 20 minutes, using a serum sample of 25 L.

In the early 1990s, it was demonstrated that measuring the ratio of free to total PSA could further help in distinguishing prostate cancer from benign prostate disease. The Hybritech free PSA testalso the first test of its kind to receive approvalhelps determine the percent of free PSA and thereby improve the accuracy of prostate cancer detection while eliminating a projected 20 percent of all prostate biopsies . The free PSA test is used following a non-suspicious DRE and a total PSA test that shows moderately elevated PSA levels (between four and 10 ng/mL) in men aged 50 years and older.

Analytical sensitivity for this assay at <0.005 ng/mL ; measurement range to 20 ng/mL and precision at UniMed LLC, Natick, Mass., manufactures a Prostate Specific Antigen (PSA); Prostate Cancer Test collection kit that includes a fingertip blood sample stylet, specimen card. The user sends the prepaid mailer to the companys laboratory where it is analyzed using the Tandem PSA method for total PSA. The results are returned by mail.

Craig Medical Distribution, Inc., Vista, Calif., produces another FDA-approved sample collection kit: the PSA4 Screening Test, a quantitative assay for a whole blood sample. As with the UniMed product, a fingertip blood sample is enclosed in a pre-addressed, postage paid mailer and sent to the laboratory where the sample is tested, the results are tabulated in report form, reviewed by a physician and returned within five days.

Given the widespread prevalence of prostate cancer the search for markers more accurate than PSA are now being widely sought. Prior to their availability, PSA testing, while in a far more limited role, will continue to perform useful work.

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