Comparative study of assays for prostate-specific antigen molecular forms

Background:The detection of prostate-specific antigen (PSA) molecular forms such as free PSA and PSA-ACT and the use of PSA molecular ratios, especially the percentage of free to total PSA, have been reported to improve the diagnostic accuracy of prostate cancer in the patients with slightly elevated serum PSA values. However, the correlation among the values of serum free PSA or PSA-ACT obtained in various assays remains unclear. Methods: Serum free PSA and PSA-ACT values were detected with the following assays: Hybritech, DPC, EIKEN, Abbott, Roche and Can-Ag for free PSA and Dainippon, Chugai, EIKEN and Bayer for PSA-ACT The data obtained with each assay were compared with Hybritech (free PSA) and Dainippon (PSA-ACT) as standards. Results: The free PSA data obtained with the Hybritech, EIKEN, Abbott and Can-Ag kits were similar. The values obtained with the DPC and Roche kits showed a linear regression of y = ax + b with those obtained with the Hybritech, with a b value of zero and an a value of 1.20 and 1.57, respectively. The serum PSA-ACT values detected with the Dainippon and Chugai kits were identical. The equation for converting the data obtained with the EIKEN kits to the Dainippon value was 0.7636 x (EIKEN) + 0.1381. Conclusions: Serum free PSA and PSA-ACT values obtained with various assays were not necessarily the same. Some kits for the assay of free PSA and PSA-ACT gave the same serum values. The free PSA values obtained with the other kits could be converted using appropriate equations. The gamma-Sm values showed wide variations and were not considered suitable as a measurement of free PSA.