Current status and future directions in standardization of prostate-specific antigen immunoassay

Redesigned proficiency testing materials improve survey outcomes for prostate-specific antigen. A College of American Pathologists Ligand Assay Survey tool

CONTEXT

Large disparities in prostate-specific antigen (PSA) results from different assays have been observed in the College of American Pathologists (CAP) Ligand Assay Survey, with interassay results varying severalfold. Survey specimens are predominately composed of free PSA and do not reflect the composition of typical patient specimens.

OBJECTIVES

To characterize a pilot material developed for CAP in which pooled sera samples were spiked with purified PSA and alpha(1)-antichymotrypsin-bound PSA at targeted concentrations and to compare it to CAP survey and reference materials.

DESIGN

CAP survey, reference, and pilot materials were analyzed using 10 total PSA and 7 free PSA assays. These assays included Food and Drug Administration-approved assays and assays for research use only.

RESULTS

Variability among the 10 total PSA methods was greatest for the 1997 ligand survey material (CV range, 56%-65%) followed by the pilot material (CV range, 10%-29%) and the reference material (CV range, 6%-13%). In contrast, interassay variability for the 7 free PSA methods was similar for the 3 preparations, with the exception of one specimen close to the limit of detection of the assays. As determined with the Hybritech Tandem-R method, the ligand survey specimens were essentially composed of all free PSA, whereas the reference and pilot materials were composed of approximately 10% and 35% free PSA, respectively.

CONCLUSIONS

The newly formulated pilot material prepared using a human base that contained defined concentrations of free PSA and alpha(1)-antichymotrypsin-bound PSA more closely resembled patient specimens and minimized differences among methods compared with the semen-supplemented original survey material.

New developments in the standardization of total prostate-specific antigen

OBJECTIVE

Analytical evaluation of the calibration of three recently launched assays for the measurement of total prostate-specific antigen, i.e., IMx Total PSA (Abbott), Elecsys PSA (Roche), and IMMULITE 3rd Generation PSA (DPC).

DESIGN AND METHODS

For accuracy assessment two reference materials were applied namely, Stanford 90:10 PSA Calibrator and Certified Reference Material 613 Prostate-Specific Antigen. Dilutions of these preparations were analyzed with all assays. In addition, clinical specimens from known prostate cancer or benign prostate hyperplasia patients and samples taken from an ongoing prostate cancer screening study were used for comparison.

RESULTS

Application of the Stanford Calibrator revealed results well within 10% of the calculated values for all assays. Regarding the CRM Calibrator only the IMx Total PSA proved to approach the line of identity. The IMMULITE results differed about 40% and the Elecsys about 18% from the calculated values. The comparison with clinical specimens showed statistically different results for the combination IMMULITE-IMx and for IMMULITE-Elecsys. The regression lines for both collections were: y(IMx) = 0.86x(IMMULITE) +0.12 (n = 104, r = 0.970, Sy/x = 0.883 microg/L) and y(Elecsys) = 0.98x(IMMULITE) +0.38 (n = 97, r = 0.976, Sy/x = 0.733 microg/L). In the lower measuring range (PSA <5.0 microg/L) as measured with the screening samples (n = 43), these differences were less pronounced.

CONCLUSION

In analytical sense a difference was found for both reference preparations in the assays studied. Clinically, despite improvements in methodology, results for total prostate-specific antigen are still not interchangeable. The possible consequences need to be elaborated.