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

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

Because of the difficulty in distinguishing between prostate malignanciesand benign prostatic hypertrophy (BPH) with PSA levels of 4 to10 ng/mL, the specificity of PSA testing is currently limited.

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

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

A first trial of 105 patients with prostate pathology (prostatecancer 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 ofF-PSA measurement and the ratio of F-PSA to total PSA yieldeda PPV of 87%.

The CIS bio international kit is available in Europe, and it holdspromise as a way to improve the specificity of PSA testing asa 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 WashingtonUniversity of St. Louis.]