A guide to the interpretation of serum prostate specific antigen levels

OBJECTIVE

To review the factors that affect the concentration of prostate specific antigen (PSA) in the serum.

RESULTS

The discussion includes the structure of PSA; its distribution and metabolism; various analytical aspects of PSA measurements; the effects of clinical manipulations on PSA, including digital rectal examination, transrectal ultrasound, cystoscopy, biopsy and transurethral resection of the prostate; factors affecting PSA levels in health, in benign disease, and in prostate cancer; the effect of various treatments on PSA; and the issue of reference ranges.

CONCLUSION

Laboratory staff and physicians must take many factors into consideration when interpreting PSA results.

PSA immunoreactivity detected in LNCaP cell medium, breast tumor cytosol, and female serum

We made an effort to identify a reliable source for obtaining large quantities of both free (PSA) and PSA-ACT complex for the preparation of the calibrator for the PSA assay. Using size exclusion chromatography, we found both free PSA and PSA-ACT complex in the conditioned cell medium of the LNCaP cell line, which was derived from a human metastatic adenocarcinoma of the prostate. An assay specific for PSA-ACT reacted only with the PSA-ACT complex from cells grown in serum-free medium, and not with the complex from the cell medium grown in 10% calf serum. We also found both free PSA and PSA-ACT complex in 15% of cytosols prepared from breast tumor tissues; the cytosol PSA concentrations ranged from 0.1 to 110 ng/ml. No correlation was found between cytosol PSA and concentrations of estrogen receptor, progestin receptor, epidermal growth factor receptor, cathepsin D, or the ectodomain of c-erbB-2 protein. Based on chromatographic characterizations and the slope of their dose-response curves, it appears that both free PSA and PSA-ACT complex found in the cytosols are similar to PSA complex from the cell medium and the serum of prostate cancer patients. Ectopic PSA was also detected in pooled sera from patients with breast, ovarian, pancreatic, and colon carcinoma. The PSA concentrations in these serum pools increased with the level of their dominant tumor marker. In any event, the LNCaP cell medium appears to be a reliable source for obtaining both free and ACT-complexed PSA of human tumor origin for the preparation of PSA assay calibrators.

Prostatic specific antigen

PSA is a 34-kDa 240-amino-acid glycoprotein produced exclusively by prostatic epithelial cells. PSA is a serine protease, is a member of the kallikrein gene family, and has a high sequence homology with human glandular kallikrein. It has chymotrypsin-, trypsin-, and esterase-like activities. In the serum it is present mainly in a complex form with alpha 1-antichymotrypsin. It is secreted in the seminal plasma and is responsible for liquefaction of the seminal coagulum. The production of PSA proteins appears to be under the control of circulating androgens acting through the androgen receptors. The PSA gene is up-regulated predominantly by androgens at both the protein and mRNA levels. DRE causes minimal changes in the PSA level, while prostate massage, ultrasonography, systoscopic examination, and prostate biopsy can all cause clinically significant elevations. Other conditions, such as prostatitis, prostate intraepithelial neoplasia, acute urinary retention, and renal failure can also elevate the PSA level. The value of PSA as a screening tool is questionable because of the great deal of overlap in PSA levels between BPH and prostate cancer. However, if used in men over 50, in conjunction with DRE and/or ultrasonography, it may become a vital part of the early detection program. PSA's role in determining the clinical and pathological stage is also limited, in spite of the direct correlation between the pathological stage and the PSA level, because of great overlap in the PSA levels in various stages. The most important clinical utility of PSA is in monitoring patients after definitive therapy. PSA is most sensitive and reliable in the detection of a residual tumor, possibly recurrence, or disease progression following treatment, irrespective of the treatment modality. PSA can accurately predict the tumor status and can detect recurrence several months before its detection by any other method. PSA is also a very sensitive and specific immunohistochemical marker for tumors of prostatic origin. Compared to PAP, PSA is a more precise and meaningful marker in all clinical situations. With the development of ultrasensitive assays and the adoption of an international standard PSA calibrator, so that results from multicenter studies can be compared, PSA could become one of the most useful tumor marker in cancer biology.

Inter-relation between measurement of serum prostatic specific antigen and transrectal ultrasound in the diagnosis of benign prostatic hyperplasia and prostatic cancer

Serum prostatic specific antigen (PSA) and ultrasound-determined prostatic volume (UPV) were measured in 50 patients with histologically proven benign prostatic hyperplasia (BPH) and in 40 patients with histologically proven prostatic cancer of whom 17 had evidence of distant metastases (M1) and 23 did not (M0). A good correlation between log PSA and UPV was demonstrated in the BPH group and rearrangement of the linear regression equation allowed calculation of a single variable--the log PSA corrected to a standard prostate volume for any given individual. A volume-corrected PSA correctly identified all patients with M1 disease and greatly improved but did not eliminate overlap of M0 disease with BPH. Reduction of serum PSA to a single volume-corrected variable will allow the introduction of practical and optimum protocols for the management of patients with prostatic enlargement.

Prostate-specific antigen: problems in analysis

We have compared an "in-house" Tenovus Institute prostate-specific antigen (PSA) assay with four different commercial kits (ELSA-PSA, IRMA-Count PSA, PROS-CHECK PSA and TANDEM-R PSA) that are available in the UK. There was only good correlation and linear regression parameters between the in-house assay and one of the kit methods. The difference in values for the same sample ranged from 2 to 100-fold. These discrepancies are due, in part, to the specificity of the polyclonal and monoclonal antibodies used in the procedures and the differing "hook effects" caused by the binding capacity of the antibody pairs in the immunometric assays. Discrepancies will, however, result from the differing potencies of the standards used for the calibration curves. This data highlights the urgency for the introduction of an internationally accepted reference standard for PSA.

Comparison of a polyclonal and monoclonal immunoassay for PSA: need for an international antigen standard

Two leading commercial immunoassays for prostate specific antigen (the Yang Laboratories polyclonal radioimmunoassay and the Hybritech two-site monoclonal radioimmunometric assay) were compared using independently purified antigen as the calibrator. The Yang polyclonal assay yielded values from 1.4 to 1.9 times higher than the Hybritech monoclonal assay on the same sample, similar to results of previous investigators. When an independent prostate specific antigen calibrator was substituted for the respective kit calibrators, both assays yielded essentially identical results, thereby confirming that differences in assay values arise from differences in the assigned values of the kit calibrators. Otherwise, both assays had similar responses in the high and low range. Recovery studies of prostate specific antigen in various diluents demonstrated that antigen recovery was lowest in diluents with the highest protein content. We conclude that the kit manufacturers have assigned different values to their antigen calibrators. Sample values between the assays can be compared by use of an appropriate conversion table. Differences in their recommended ranges of normal for prostate specific antigen in male serum (zero to 2.5 ng./ml. for the Yang assay and zero to 4.0 ng./ml. for the Hybritech assay) reflect two factors: a difference in assigned calibrator values and differences in the selection of the "normal" population of men used to define the normal range of values. We urge adoption of an international standard calibrator for prostate specific antigen as a means to correct these differences.

Clinical use of prostate specific antigen in patients with prostate cancer

The clinical use of prostate specific antigen as a screening test for prostate cancer, as a preoperative determinant for staging of prostate cancer and to monitor response to therapy in prostatic cancer patients was evaluated in 168 men with benign prostatic hyperplasia and 231 men with prostate cancer. Only 3% of the men with benign prostatic hyperplasia had prostate specific antigen levels greater than 10 ng. per ml. compared to 44% of the men with proved prostate cancer. Preoperative prostate specific antigen levels increased with higher clinical stages of prostate cancer but there was substantial overlap among stages. Among patients with stage A1 prostate cancer who were followed expectantly none had an elevated prostate specific antigen value or metastatic disease during a followup of 15 to 120 months. After radical prostatectomy serum prostate specific antigen values decreased to undetectable levels (less than 0.6 ng. per ml.) in 89% of the patients with organ-confined disease, in 87% of those with microscopically positive margins only but in only 34% with seminal vesicles or lymph node involvement. Failure of the prostate specific antigen levels to decrease to the undetectable range after radical prostatectomy was associated with a greater likelihood of subsequent tumor recurrence. Only 3 of 18 patients (17%) treated with definitive radiation therapy had post-irradiation prostate specific antigen values of less than 0.6 ng. per ml., while in 39% the prostate specific antigens values remained greater than 4 ng. per ml. and in 4 of 18 (22%) the values were greater than 10 ng. per ml. Of patients with previously untreated stage D2 prostate cancer the mean pre-treatment prostate specific antigen value was 63.7 ng. per ml. compared to a post-hormonal therapy mean value of 31.1 ng. per ml. Of 32 patients treated with hormonal therapy 14 had stable disease, including 13 with prostate specific antigen levels of less than 10 ng. per ml. In contrast, 18 patients had progressive disease, of whom 16 had prostate specific antigen levels of more than 10 ng. per ml. We conclude that the serum prostate specific antigen assay is most useful clinically to monitor the response to therapy of prostate cancer patients.

Prostate specific antigen: not detectable despite tumor progression after radical prostatectomy

Prostate specific antigen was not detectable in 2 men with documented prostate cancer progression after radical prostatectomy. Possible explanations for prostate specific antigen remaining zero in these situations are discussed. We conclude that while monitoring prostate specific antigen is of great value in the followup of patients with prostatic cancer, it has not replaced more standard means of followup.