Is the WHO 90:10 Prostate-Specific Antigen (PSA) First International Reference Standard Really 90% α1-Antichymotrypsin–Bound PSA and 10% Free PSA?

Abstract

To the Editor: Several factors can contribute to differences among immunoassay methods. Such differences have led to a recognition of the need for the standardization of prostate-specific antigen (PSA)1 assays, and 2 Stanford conferences on the international standardization of PSA immunoassays were held in the early 1990s (1, 2). In 1999, the WHO Expert Committee on Biological Standardization established the First International Standards for PSA with materials that emanated from recommendations of these conferences. These materials consisted of lyophilized 1-μg preparations of 100% free PSA (96/668), and 90% α1-antichymotrypsin–bound PSA (PSA-ACT) and 10% free PSA (96/670) (3) (www.nibsc.ac.uk). A preliminary study found that when total and free PSA were measured in solutions of the 90% PSA-ACT and 10% free PSA (90:10 PSA) material from Stanford/WHO, the slope of the regression line that compared the measured free PSA to measured total PSA was 0.15 instead of the expected 0.10 (unpublished data). The objective of the current study was to further investigate this observation by comparing PSA standard materials from 2 sources (Stanford/WHO and Scripps Laboratories) in 4 total and free PSA assays. The Stanford/WHO reference materials (PSA-ACT standard and 90:10 PSA standard; lyophilized, 1 μg/vial) were purchased from Stanford University. Vials were reconstituted with 2 mL deionized water. Free PSA and …