Effect of digital rectal examination and flexible cystoscopy on free and total prostate-specific antigen, and the percentage of free prostate-specific antigen. Differences between two PSA assays

OBJECTIVE
To determine the effect of prostate manipulations on free PSA, total PSA and the percentage of free PSA using an equimolar and nonequimolar PSA assays.

METHOD
A total of 67 men were studied. Blood samples were obtained before and 45-60 min after two different prostatic manipulations: DRE from 45 patients and flexible cystoscopy from 22 patients. Total PSA (t-PSA) was assayed with a non equimolar method using the Cobas Core PSA kit from Roche (Cobas Core-PSA) and an equimolar method using the Immulite 3rd-generation PSA kit from DPC (Immulite-PSA). Free PSA (f-PSA) was quantified with an Immulite free PSA kit. We obtained two f-PSA/t-PSA ratios dividing the f-PSA by both t-PSA (Cobas Core-PSA) and t-PSA (Immulite-PSA).

RESULTS
The Cobas Core-PSA yielded higher values than Immulite-PSA and the baseline percentages of free PSA were different depending on the method used. Both t-PSA values were increased after DRE and flexible cystoscopy from the baseline although the increase of Cobas Core-PSA was much higher than Immulite-PSA. Also the f-PSA was increased after the two manipulations. The f-PSA/t-PSA ratio (Cobas Core-PSA) remains at the baseline whereas the f-PSA/t-PSA ratio (Immulite-PSA) had an important increase from the baseline.

CONCLUSION
The effect of DRE and flexible cystoscopy is different depending on the PSA assay used. The data from studies which are carried out in relation with the molecular forms of the PSA are not interchangeable if they are performed with different PSA methods (equimolar and nonequimolar). The serum samples should precede any prostate manipulation in all investigations related to PSA molecular forms.

CITATION Eur Urol. 1998;33(3):255-60