A new modification of the chiron ACS assay for total prostate-specific antigen achieves equimolar response characteristics and improves the detection of prostate cancer

Nonequimolar-response assays for prostate-specific antigen (PSA)
are criticized for overestimating total PSA in some men without
prostate cancer (PCA), and underestimating total PSA in some men
with PCA. We recently studied three nonequimolar-response PSA
assays that had undergone modifications. While two of the studied
assays achieved equimolar-response characteristics with improved
areas under receiver operating characteristic (ROC) curves (AUC),
the modification of the Chiron ACS PSA assay (ACS PSA2, Chiron)
failed to achieve this. Recently, the ACS assay underwent another
modification (ACS PSA, Bayer), which we investigated. Sera from 305
men (155 without and 150 with PCA, PSA greater than or equal to2
and less than or equal to30 mug/l, TandemE) were measured using
both modifications of the ACS assay and equimolar-response
reference methods (TandemR free and Tandem E, Hybritech). Molar
response relative to the reference method and clinical performance
(comparison of AUCs) between the previous and new ACS assay
modifications were studied. The new modification of the ACS assay
(ACS PSA, Bayer) achieved equimolar-response characteristics but
reported lower values (average 10%) than the Tandem E assay.
Compared to the previous modification (ACS PSA2, Chiron), a 3%
improvement in AUC (p=0.01) was found. Using results of the
redesigned equimolar-response assay (ACS PSA, Bayer), we calculated
that 6 of 155 men without PCA in this sample set could be spared
unnecessary biopsy compared with the previous nonequimolar-response
assay (ACS PSA2, Chiron) without missing additional PCA (90%
sensitivity). These data provide additional evidence for clinical
advantages of equimolar-response over nonequimolar-response PSA
assay formats.