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New Assay Measures PSA/Free-PSA Ratio, Shows Improved Specificity

New Assay Measures PSA/Free-PSA Ratio, Shows Improved Specificity

NEW YORK--A new testing kit that measures free PSA (F-PSA), developed by a French pharmaceutical firm, may improve the positive predictive value (PPV) of PSA testing.

Because of the difficulty in distinguishing between prostate malignancies and benign prostatic hypertrophy (BPH) with PSA levels of 4 to 10 ng/mL, the specificity of PSA testing is currently limited.

At the International Conference on Human Tumor Markers, Nathalie Bedeau, of CIS bio international (Gif-sur-Yvette), described a new assay that measures the ratio of total PSA to F-PSA.

Ms. Bedeau explained that PSA assays currently in use measure total PSA without making a distinction between F-PSA and PSA bound to alpha-1-antichymo-trypsin. It is known that the proportion of F-PSA is higher in patients with BPH than in those with prostate cancer. The ability to measure both total PSA and F-PSA, and to derive a ratio between the two, has been shown to nearly double the positive predictive value of PSA testing.

A first trial of 105 patients with prostate pathology (prostate cancer or BPH) showed that total PSA (with a cut-off of 4 ng/mL) yielded a PPV of only 48%. In the same population, the use of F-PSA measurement and the ratio of F-PSA to total PSA yielded a PPV of 87%.

The CIS bio international kit is available in Europe, and it holds promise as a way to improve the specificity of PSA testing as a diagnostic tool, Ms. Bedeau commented.

[Editors' note: In the US, Hybritech has developed a similar assay, to be tested in a large clinical trial coordinated by Washington University of St. Louis.]

 
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