ASSAY VARIABILITY IN SERUM PROSTATE-SPECIFIC ANTIGEN DETERMINATION

Consistency and reproducibility of serum prostate-specific antigen (PS A) measurement are essential in the application of this analyte to ear ly detection or screening programs. In the present investigation, we s ought to compare serum PSA levels determined by the IMx assay (Abbott Laboratories, North Chicago, IL) and the Tandem E (Hybritech Inc., San Diego, CA) to determine whether there were differences. Two hundred t wenty-eight random sera from our archival bank were investigated. One hundred-eight specimens were in the Tandem range of 2.0-10.0 ng/ml, an d prostatic histology was known based on either systematic sector need le biopsy or transurethral resection. PSA was measured with three diff erent lots of the IMx and Tandem assays. Over the entire range, there was a good correlation (r(2) = 0.985); however, in the more useful cli nical range of 2.0-10.0 ng/ml, the correlation was reduced to 0.923; i n the 2.0-6.0 ng/ml range, it was further reduced to 0.852. The slope for the entire range was 0.948; however, in the 2.0- to 10.0-ng/ml ran ge, it was 0.894; in the 2.0- to 6.0-ng/ml range, the slope was 0.815. Using PSA cutoffs of 4.0, 5.0, and 6.0 ng/ml, significant decrease in abnormal PSA values in men with cancer was observed with the IMx comp ared with Tandem. These data suggest that the IMx and the Tandem PSA a ssays are not equivalent, and in most patients a lower value is realiz ed with the IMx assay. This bias appears to be greater in men with pro state cancer. Clinicians must be aware which assay their patients are being tested with, and laboratory technicians should run internal stan dards to ensure lack of significant intralot variability and consisten cy over time. (C) 1995 Wiley-Liss, Inc.