Molecular forms of serum prostate-specific antigen. The clinical value of percent free prostate-specific antigen

The concept of measuring the proportions of various forms of PSA in serum, particularly the proportion of free to total PSA, represents a new and exciting method of detecting early curable prostate cancers and avoiding unnecessary prostate biopsies in men who have BPH only. Compared with other methods of improving diagnostic specificity, it does not require transrectal ultrasound for determination of prostate volume, as does the use of PSA density, and it does not require multiple blood sampling over a sufficiently long period, as does PSA velocity. Recent findings suggest determination of the proportion of free to total PSA, rather than that of complexed to total PSA, to be the optimal discriminator between patients with prostate cancer and those with BPH in the PSA reflex range of 2.5 or 3 ng/mL to 10 ng/mL, and to improve the clinical accuracy of the PSA test substantially. If the total PSA value is normal, percent free PSA improves the sensitivity (increases cancer detection) of the PSA test; if the total PSA value is slightly elevated, percent free PSA enhances the specificity (eliminates unnecessary negative prostate biopsies) of the PSA test. Both of these outcomes are clinically desirable in attempting to diagnose early, curable prostate cancers in a cost-effective manner among men who also have varying degrees of BPH. Figure 5 contains a diagnostic algorithm for the detection of clinically significant prostate cancers at a curable stage, employing the concept of percent free PSA. As more is learned about percent free PSA, however, it may be necessary to make modifications in how this concept is used clinically.