Improving the comparability of immunoassays for prostate-specific antigen (PSA): Progress and problems

Verification of Harmonization of Serum Total and Free Prostate-Specific Antigen (PSA) Measurements and Implications for Medical Decisions

BACKGROUND

Previous studies have shown that the harmonization of prostate-specific antigen (PSA) assays remained limited even after the introduction of WHO International Standards. This information needs updating for current measuring systems (MS) and reevaluation according to established analytical performance specifications (APS) and the characteristics of antibodies used.

METHODS

Total (tPSA) and free (fPSA) PSA were measured in 135 and 137 native serum samples, respectively, by Abbott Alinity i, Beckman Access Dxl, Roche Cobas e801, and Siemens Atellica IM MSs. Passing-Bablok regression and difference plots were used to compare results from each MS to the all-method median values. Agreement among methods was evaluated against APS for bias derived from biological variation of the 2 measurands.

RESULTS

The median interassay CV for tPSA MSs (11.5%; 25-75th percentiles, 9.2-13.4) fulfilled the minimum APS goal for intermethod bias (15.9%), while the interassay CV for fPSA did not [20.4% (25-75th percentiles, 18.4-22.7) vs goal 17.6%]. Considering the all-method median value of each sample as reference, all tPSA MSs exhibited a mean percentage bias within the minimum goal. On the other hand, Alinity (+21.3%) and Access (-24.2%) were out of the minimum bias goal for fPSA, the disagreement explained only in minimal part by the heterogeneity of employed antibodies.

CONCLUSIONS

The harmonization among tPSA MSs is acceptable only when minimum APS are applied and necessitates further improvement. The marked disagreement among fPSA MSs questions the use of fPSA as a second-level test for biopsy referral.

Is pre-biopsy serum prostate specific antigen retesting always justified? A study of the influence of individual and analytical factors on decision making for biopsy referral

BACKGROUND AND AIMS

We investigated factors influencing pre-biopsy prostate-specific antigen (PSA) retesting as recommended by clinical guidelines.

MATERIALS AND METHODS

333 patients screened for prostate cancer (PCa) repeated PSA (Roche Cobas systems) after a median of 3.9 months, before performing biopsy. Multiple regression models were used to assess effects of patients' characteristics on PSA results and changes over time.

RESULTS

PCa [n = 132 (40.7%)] and cancer-free [n = 192 (59.3%)] patients had similar rate of PSA positive results at baseline (84.8% vs. 83.9%, P = 0.931). Their rate of reversion to normal PSA after retesting was negligible (0.9% in PCa and 3.7% in PCa-free patients, P = 0.286). 31.1% of PCa and 31.3% of cancer-free patients (P = 0.426) showed a significant PSA increase after retesting. Age was a confounder since not only PSA increased in older PCa patients, but it was also related to PCa histological grade, in turn associated to PSA increase. In PCa-free patients, glandular inflammation, present in 1/3 of subjects, was also associated to higher PSA concentrations.

CONCLUSION

When obtained with the same immunoassay under controlled analytical conditions, a PSA positive result is confirmed after retesting in the great majority of screened patients. Neither analytical factors nor intraindividual variability appeared to justify PSA retesting before biopsy referral.

Systematic Differences Between Total and Free Prostate-Specific Antigen Immunoassays: Comparison Using Passing and Bablok Regression

Recent studies have shown that there are systematic differences among total and free prostate-specificantigen (PSA) immunoassays. In this study we analyzedintermethod differences in total PSA (tPSA) and free PSA(fPSA) measurement using ARCHITECT i2000SR (Abbott Diagnostics) and COBAS E601 (Roche Diagnostics). A number of 160 blood samples were tested for tPSA and 50 samples for fPSA (selecting only sampleswith tPSA: 4.1-10.0 μg/L). Passing-Bablok regression analysis was used to compare the two analytical methods fortPSA, fPSA and percentage of fPSA (%fPSA). A strong correlation was noticed between ARCHITECT i2000SR and COBAS E601 for tPSA, fPSA and %fPSA (r between 0.94 and 0.99). Concentrations of tPSA and fPSA measured by COBAS E601 were higher thanthose measured by ARCHITECT i2000SR with a bias of 0.8 μg/L for tPSA and 0.14 μg/L for fPSA. Analyzing therelative difference between methods for fPSA and %fPSA, COBAS E601 exceed a 10% relative difference limit. Our study confirms that there are differences in measured concentrations of tPSA and fPSA byvarious commercial methods. Because clinical judgment on subsequent diagnostic procedures, such as prostatebiopsy, is based on tPSA and fPSA results, tests harmonization should be a priority.

20–25% Lower Concentrations of Total and Free Prostate-Specific Antigen (PSA) after Calibration of PSA Assays to the WHO Reference Materials – Analysis of 1098 Patients in Four Centers

Aim

To examine the potential clinical implications of the recalibration of total prostate-specific antigen (PSA) and free PSA (fPSA) assays to the World Health Organization (WHO) standard materials.

Material and methods

Data from 1098 patients with or without clinically detected prostate cancer (PCa) from four independent cohort studies were compared using commercial assays calibrated to the traditional Hybritech® PSA (PSA-Hyb) and fPSA (fPSA-Hyb) standards and to the WHO 96/670 (PSA-WHO) and 96/668 (fPSA-WHO) standards. The Access® Immunoassay System (Beckman Coulter, Inc.) was used in all studies.

Results

All studies showed 20% to 25% lower PSA and fPSA test results with the WHO-standardized assays. No significant change in %fPSA (fPSA/PSA × 100) was observed. Continuing to use the traditional clinical PSA cutoffs obtained with the Hybritech standard after changing to the PSA-WHO standard could result in up to one-third of prostate cancer cases being missed. Conclusions: Manufacturers should fully inform laboratories about a calibration change and its clinical impact. Laboratory reports for PSA measurements should indicate the assay's manufacturer and which calibration standard was used to avoid misleading information concerning PCa risk

Suitability of quality control materials for prostate-specific antigen (PSA) measurement: inter-method variability of common tumor marker control materials

BACKGROUND

Quality control materials with minimal inter-assay differences and clinically relevant proportions of different molecular forms of the analyte are needed to optimize intra- and inter-laboratory accuracy and precision.

METHODS

We assessed if clinically relevant total prostate-specific antigen (tPSA) levels were present in seven commercially available Multi Constituent Tumor Marker Controls (MC-TMC). Further, we determined the concentration of free PSA (fPSA) and calculated the percentage of free PSA (%fPSA) in all materials. Finally, we determined variability of TMC materials across several commonly used PSA platforms.

RESULTS

All MC-TMC materials contained at least one concentration of tPSA in normal and pathologic range. Control materials varied in the amount of fPSA and %fPSA, with most controls consisting of fPSA only and only one MC-TMC containing medically relevant levels of around 35% fPSA. Only a minority of MC-TMC materials showed minimal variability across four PSA methods while the majority of PSA controls showed wide inter-method differences.

CONCLUSIONS

Use of many commercially available controls for PSA could lead to biased PSA measurements because they contain medically irrelevant proportions of fPSA and show significant variation among different PSA assay platforms.

National Academy of Clinical Biochemistry Laboratory Medicine Practice Guidelines for Use of Tumor Markers in Testicular, Prostate, Colorectal, Breast, and Ovarian Cancers

Background: Updated National Academy of Clinical Biochemistry (NACB) Laboratory Medicine Practice Guidelines for the use of tumor markers in the clinic have been developed.

Methods: Published reports relevant to use of tumor markers for 5 cancer sites—testicular, prostate, colorectal, breast, and ovarian—were critically reviewed.

Results: For testicular cancer, α-fetoprotein, human chorionic gonadotropin, and lactate dehydrogenase are recommended for diagnosis/case finding, staging, prognosis determination, recurrence detection, and therapy monitoring. α-Fetoprotein is also recommended for differential diagnosis of nonseminomatous and seminomatous germ cell tumors. Prostate-specific antigen (PSA) is not recommended for prostate cancer screening, but may be used for detecting disease recurrence and monitoring therapy. Free PSA measurement data are useful for distinguishing malignant from benign prostatic disease when total PSA is <10 μg/L. In colorectal cancer, carcinoembryonic antigen is recommended (with some caveats) for prognosis determination, postoperative surveillance, and therapy monitoring in advanced disease. Fecal occult blood testing may be used for screening asymptomatic adults 50 years or older. For breast cancer, estrogen and progesterone receptors are mandatory for predicting response to hormone therapy, human epidermal growth factor receptor-2 measurement is mandatory for predicting response to trastuzumab, and urokinase plasminogen activator/plasminogen activator inhibitor 1 may be used for determining prognosis in lymph node–negative patients. CA15-3/BR27–29 or carcinoembryonic antigen may be used for therapy monitoring in advanced disease. CA125 is recommended (with transvaginal ultrasound) for early detection of ovarian cancer in women at high risk for this disease. CA125 is also recommended for differential diagnosis of suspicious pelvic masses in postmenopausal women, as well as for detection of recurrence, monitoring of therapy, and determination of prognosis in women with ovarian cancer.

Conclusions: Implementation of these recommendations should encourage optimal use of tumor markers.

Toward metrological traceability in the determination of prostate-specific antigen (PSA): calibrating Beckman Coulter Hybritech Access PSA assays to WHO standards compared with the traditional Hybritech standards

BACKGROUND

The metrological traceability of prostate-specific antigen (PSA) assay calibration to WHO standards is desirable to potentially improve the comparability between PSA assays. A method comparison was performed between the traditionally standardized Beckman Coulter Hybritech Access PSA and free PSA (fPSA) assays and a new alternate calibration of assays aligned to the WHO standards 96/670 and 96/668, respectively.

METHODS

Sera from 641 men with and without prostate cancer, various control materials and mixtures of different proportions of the WHO standards were measured with both assay calibrations.

RESULTS

Excellent comparability between the corresponding assay calibrations was observed, with correlation coefficients of at least 0.996. The Passing-Bablok slopes were 0.747 for total PSA (tPSA), 0.776 for fPSA and 1.02 for the percentage ratio of fPSA to tPSA (%fPSA), while the corresponding percentages of the new WHO-aligned assay results related to the traditional assays were 76.2%, 77% and 102.2%. Receiver operating characteristics revealed no differences between the two PSA assay calibrations.

CONCLUSIONS

The WHO calibration yields results approximately 25% lower for tPSA and fPSA values when compared with the conventional Hybritech calibration. Using the WHO-aligned PSA assay, a tPSA cut-off of 3 microg/L should be considered in clinical practice, while %fPSA cut-offs could be retained.

Quantitative and label-free technique for measuring protease activity and inhibition using a microfluidic cantilever array

We report the use of a SiN x based gold coated microcantilever array to quantitatively measure the activity and inhibition of a model protease immobilized on its surface. Trypsin was covalently bound to the gold surface of the microcantilever using a synthetic spacer, and the remaining exposed silicon nitride surface was passivated with silanated polyethylene glycol. The nanoscale cantilever motions induced by trypsin during substrate turnover were quantitatively measured using an optical laser-deflection technique. These microcantilever deflections directly correlated with the degree of protease turnover of excess synthetic fibronectin substrate ( K M = 0.58 x 10 (-6) M). Inhibition of surface-immobilized trypsin by soybean trypsin inhibitor (SBTI) was also observed using this system.