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, especial ly the percentage of free to total PSA, have been reported to improve the d iagnostic accuracy of prostate cancer in the patients with slightly elevate d 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 D ainippon, Chugai, EIKEN and Bayer for PSA-ACT The data obtained with each a ssay were compared with Hybritech (free PSA) and Dainippon (PSA-ACT) as sta ndards. Results: The free PSA data obtained with the Hybritech, EIKEN, Abbott and C an-Ag kits were similar. The values obtained with the DPC and Roche kits sh owed a linear regression of y = ax + b with those obtained with the Hybrite ch, with a b value of zero and an a value of 1.20 and 1.57, respectively. T he serum PSA-ACT values detected with the Dainippon and Chugai kits were id entical. 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 othe r 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.