The pPSA forms are more highly enriched in prostate tumors and are a more cancer specific marker of prostate cancer. The pPSA forms are especially useful in the 2.5-4 ng/ml PSA range, where the other PSA forms show little diagnostic utility. Serum pPSA, as a percentage of free PSA, has a higher specificity for cancer than the individual PSA assays or ratios of free, complexed or total PSA. The ratio of pPSA and FPSA may also provide a more stable parameter than indi-vidual measurements of FPSA or cPSA.⁴² In Fig. 3 the %[-2]pPSA is shown, but %pPSA, (the sum of all three pPSA forms) gave similar results that were superior to %FPSA and cPSA. Individual assays for the different pPSA forms have been employed in initial studies in order to determine if any of the individual pPSA forms has greater ultility than other pPSA forms. In some cases it may be desirable to measure only the sum of all pPSA forms and in this case a single research assay has also been developed that can measure all forms of pPSA.
pPSA is a subform of free PSA and the probability of cancer increases as the percentage of pPSA in the FPSA increases. This may at first seem paradoxical since the free PSA is normally associated with benign disease, but upon closer examination the presence of pPSA may help explain why free PSA is not more benign-specific. For instance, 8% of men with %FPSA greater than 25% are predicted to have prostate cancer, while 44% of the men with less than 10% free PSA will still not have cancer.¹⁴ The presence of pPSA or BPSA in the FPSA may contribute to the misdiagnosis of benign disease or cancer in some patients with only a %FPSA measurement. Preliminary evidence indicates that pPSA is significantly elevated in men with greater than 25% FPSA who have been diagnosed with cancer. Therefore pPSA may help discriminate cancer in those men with high %FPSA who would normally not be biopsied. Another area of promise for pPSA is the discrimination of aggressive cancers from more indolent cancer. In summary, the current evidence indicates that pPSA is a more cancer specific form of PSA and extends the current utility of PSA to detect prostate cancer.