Significance of free prostate-specific antigen and gamma-seminoprotein in the screening of prostate cancer

Mean sojourn time, overdiagnosis, and reduction in advanced stage prostate cancer due to screening with PSA: implications of sojourn time on screening

This study aimed to assess the mean sojourn time (MST) of prostate cancer, to estimate the probability of overdiagnosis, and to predict the potential reduction in advanced stage disease due to screening with PSA. The MST of prostate cancer was derived from detection rates at PSA prevalence testing in 43 842 men, aged 50–69 years, as part of the ProtecT study, from the incidence of non-screen-detected cases obtained from the English population-based cancer registry database, and from PSA sensitivity obtained from the medical literature. The relative reduction in advanced stage disease was derived from the expected and observed incidences of advanced stage prostate cancer. The age-specific MST for men aged 50–59 and 60–69 years were 11.3 and 12.6 years, respectively. Overdiagnosis estimates increased with age; 10–31% of the PSA-detected cases were estimated to be overdiagnosed. An interscreening interval of 2 years was predicted to result in 37 and 63% reduction in advanced stage disease in men 65–69 and 50–54 years, respectively. If the overdiagnosed cases were excluded, the estimated reductions were 9 and 54%, respectively. Thus, the benefit of screening in reducing advanced stage disease is limited by overdiagnosis, which is greater in older men.

A meta-analysis of the performance characteristics of the free prostate-specific antigen test

OBJECTIVES

To conduct a meta-analysis of the diagnostic performance of the percent free prostate-specific antigen (%fPSA) test in determining prostate cancer status and to assess its value in helping to decide whether to biopsy the prostate.

METHODS

Articles identified through a MEDLINE search were included if they presented adequate original primary data to calculate a receiver operating characteristic (ROC) curve in subjects possessing histopathologically verified diagnoses. Articles containing subjects with concurrent, non-prostate-related genitourinary conditions were excluded. Percent free PSA sensitivity, specificity, ROC curves, and positive likelihood ratios were calculated for all PSA ranges and for the reflex range of PSA between 4 and 10 ng/mL.

RESULTS

Among the 41 studies, containing 19,643 subjects, area under the curve for %fPSA was 0.70 for all PSA levels, decreasing to 0.68 in the reflex range. A test cutoff of 20% would lead to 92% sensitivity and 23% specificity. Positive likelihood ratios ranged from 1.0 to 4.0, exceeding 2.0 at %fPSA of 15% or less. Within the reflex range, however, likelihood ratios exceeded 2.0 only at %fPSA of 7% or less.

CONCLUSIONS

Percent free PSA can be a useful adjunct to PSA for primary prostate cancer screening only under certain defined situations. In the "gray zone," or reflex range, of PSA testing, %fPSA improves clinical information only when levels reach extreme values.

Ratio of gamma-seminoprotein to prostate-specific antigen for the detection of prostate cancer: its discrimination power could be influenced by the assay methods of PSA and/or gamma-seminoprotein

BACKGROUND

The ratio of gamma-seminoprotein (gamma-Sm) and prostate-specific antigen (PSA) has been regarded as being superior over PSA alone as a discriminator between prostate cancer and benign prostatic diseases. In previous studies, PSA and gamma-Sm were measured by the Eiken kit and the old-version or revised Chugai kit, respectively. We compared the power of gamma-Sm ratio with that of PSA alone when using Markit-M PSA assay and the revised Chugai gamma-Sm assay.

METHODS

Fifty-three patients with prostate cancer having no metastasis and 116 with benign prostatic diseases were enrolled in this study. Prostate-specific antigen was measured by Markit-M kit and gamma-Sm was measured by the revised Chugai kit. The discrimination power of gamma-Sm ratio and PSA alone was evaluated with receiver operating characteristic (ROC) curves. Comparisons between prostate cancer and benign diseases were performed with Mann Whitney U-test and Fisher's exact test.

RESULTS

The optimal cut-off value was set at 3.1 ng/mL for PSA and 0.935 for gamma-Sm ratio. Sensitivity, specificity and positive predictive value of PSA alone were 81.1, 81.0 and 66.2%, respectively, while those of gamma-Sm ratio were 73.6, 90.5 and 78.0%, respectively. There was no statistical significance in each value between PSA and gamma-Sm ratio. Areas under the ROC curves of PSA and gamma-Sm ratio were 0.881 and 0.866, respectively (P>0.05).

CONCLUSION

Contrary to the previous reports, gamma-Sm ratio and PSA were not different in the discrimination between prostate cancer and benign prostatic diseases, which suggested that the discrimination power of gamma-Sm ratio, and presumably that of the free PSA to total PSA ratio as well, could be considerably influenced by the assay kits for serum PSA and/or gamma-Sm (free PSA) used. Therefore, the clinical significance of gamma-Sm ratio should be evaluated for each PSA assay kit.

Human Kallikrein 2 (hK2) and prostate-specific antigen (PSA): two closely related, but distinct, kallikreins in the prostate

Recent studies on human kallikrein 2 (hK2) have revealed striking similarities and significant differences with the closely related kallikrein PSA. Both PSA and hK2 are primarily localized to the prostate and share close structural similarities. Although both kallikreins are produced by the same secretory epithelial cells in the prostate, hK2 is associated more with prostate tumors than PSA and is highly expressed in poorly differentiated cancer cells. The potent trypsin-like activity of hK2 contrasts with the weak chymotrypsin-like activity of PSA. The inactive precursor form of PSA, proPSA, is converted rapidly to active PSA by hK2, suggesting an important in vivo regulatory function by hK2 on PSA activity. The high homology between hK2 and PSA results in significant cross-reactivity to hK2 by polyclonal and some monoclonal antibodies to PSA. Future studies on both PSA and hK2 need to take into account this potential for cross-reactivity. Specific monoclonal antibodies to hK2 have now demonstrated that serum levels of hK2, like PSA, are correlated with prostate cancer. The production of hK2 protein in active protease form and specific monoclonal antibodies to the hK2 antigen will allow extensive future studies delineating the physiological and clinical utility of this new prostate antigen.