EVALUATION OF A 2-SITE IMMUNORADIOMETRIC ASSAY FOR MEASURING NONCOMPLEXED (FREE) PROSTATE-SPECIFIC ANTIGEN

Serum prostate-specific antigen (PSA) in men is present as two differe nt molecular forms separable by gel-filtration chromatography (GFC), W e have evaluated a two-site IRMA that measures only the noncomplexed ( free) form of PSA (F-PSA). Verification that the F-PSA assay measures solely F-PSA was obtained by assaying GFC-fractionated serum samples w ith both the F-PSA IRMA and a commercial PSA assay that measures total PSA (T-PSA: F-PSA plus alpha(1)-antichymotrypsin-complexed PSA), The F-PSA assay detected only the 30-kDa peak corresponding to the free fo rm of PSA, whereas the T-PSA assay detected two peaks: complexed PSA a t similar to 90 kDa and F-PSA at similar to 30 kDa, The F-PSA assay ha d an analytical detection limit of 0.03 mu g/L and a measuring range u p to 50 mu g/L. The intraassay CV was 1.7-10% in the concentration ran ge of 0.2-30 mu g/L. The interassay CV was 3.4-12.5% in the same conce ntration range, Dilution and recovery studies showed no significant de viation from linearity across the assay range, The assay was insensiti ve to interference from hemoglobin, bilirubin, and total lipids up to concentrations of 5, 0.2, and 10 g/L, respectively. No significant los s of immunological activity (analyte stability) was seen day-to-day (l ess than or equal to 5) or after repeated freeze/thaw (less than or eq ual to 5) cycles, We conclude that the F-PSA IRMA. is an accurate, pre cise, and reliable tool for measuring F-PSA in human serum.