1608 Arch Pathol Lab Med—Vol 124, November 2000 Redesigned Proﬁciency Testing Materials for PSA—Sokoll et al CAP Laboratory Improvement Programs Redesigned Proﬁciency Testing Materials Improve Survey Outcomes for Prostate-Speciﬁc Antigen A College of American Pathologists Ligand Assay Survey Study Lori J. Sokoll, PhD; David L. Witte, MD, PhD; George G. Klee, MD, PhD; Daniel W. Chan, PhD, DABCC ● 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 speci- mens. Objectives.—To characterize a pilot material developed for CAP in which pooled sera samples were spiked with puriﬁed PSA and  1 -antichymotrypsin–bound PSA at tar- geted 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–ap- proved 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 ex- ception 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 com- posed of all free PSA, whereas the reference and pilot ma- terials were composed of approximately 10% and 35% free PSA, respectively. Conclusions.—The newly formulated pilot material pre- pared using a human base that contained deﬁned concen- trations of free PSA and  1 -antichymotrypsin–bound PSA more closely resembled patient specimens and minimized differences among methods compared with the semen-sup- plemented original survey material. (Arch Pathol Lab Med. 2000;124:1608–1613) L arge disparities in prostate-speciﬁc antigen (PSA) re- sults from different assays have been observed in the College of American Pathologists (CAP) Ligand Assay Survey. Interassay results have been shown to vary sev- eralfold. 1 The PSA survey specimens have been prepared by adding semen, primarily composed of free (uncom- plexed) PSA, to a processed human plasma base. The PSA from semen was used in proﬁciency and quality control materials at that time, because it was a convenient source of concentrated PSA. In addition, the different circulating molecular forms of PSA were not well understood. Using gel-ﬁltration chromatography to fractionate sur- vey specimens, Garg et al 2 veriﬁed that most PSA in CAP survey specimens is in fact free PSA. It is now well rec- ognized that assays that do not equally recognize the free and  1 -antichymotrypsin (ACT)–bound forms of PSA can have a skewed response compared with equimolar as- says. 3 Several studies have shown that reactivities of dif- ferent PSA assays to free PSA and PSA-ACT are a major Accepted for publication May 8, 2000. From the Department of Pathology, The Johns Hopkins Medical In- stitutions, Baltimore, Md (Drs Sokoll and Chan); Laboratory Control Ltd, Ottumwa, Iowa (Dr Witte); and Department of Laboratory Medi- cine, Mayo Clinic, Rochester, Minn (Dr Klee). Reprints: Daniel W. Chan, PhD, DABCC, Department of Pathology, The Johns Hopkins Medical Institutions, 600 N Wolfe St, Meyer B-121, Baltimore, MD 21287 (e-mail: dchan@jhmi.edu). contributing factor to disparate proﬁciency survey re- sults. 2,4,5 Factors other than equimolarity can also contribute to differences among assays. These factors include the fol- lowing: assay design and kinetics, antibody speciﬁcity, and calibrator composition and assignment. 3,6–8 Recogni- tion of PSA assay differences has prompted a number of efforts to standardize PSA assays. One such effort was the development of PSA reference material by CAP following a CAP and American Cancer Society conference on PSA in 1992. 2,9 This material, prepared by pooling patient se- rum samples, was intended to reﬂect the fact that, in con- trast to the distribution of the molecular forms of PSA present in semen, PSA in blood is primarily bound to ACT (80%), whereas only a small proportion is unbound. 10,11 Analysis of the reference material revealed a percentage of free PSA of approximately 10% and close agreement among methods when these specimens were included in the 1994 CAP ligand survey. 2 The purpose of the CAP reference material was to pro- vide an assayed material representative of clinical speci- mens for laboratories and manufacturers to evaluate the performance of PSA assays and to provide materials to aid in the standardization process. 9 Following develop- ment of this material, the CAP Ligand and Therapeutic Drug Monitoring Resource Committee endeavored to in- troduce proﬁciency testing specimens for the ligand sur-