Effect of digital rectal examination on serum complexed and free prostate-specific antigen and percentage of free prostate-specific antigen.

OBJECTIVES: To evaluate the effects of digital rectal examination (DRE) on serum total, calculated complexed, and free prostate-specific antigen (PSA) and the free/total PSA ratio and factors affecting variations of PSA.

METHODS: Serum total and free PSA and the percent free PSA ratio were determined in 91 urologically referred men (mean age 64 +/- 9 years) before and 30 minutes after DRE, with the Hybritech Tandem-R assay. Complexed PSA was calculated as the difference between total and free PSA. DRE effects on PSA were analyzed comparing pre-DRE and post-DRE values and by calculating the difference between pre-DRE and post-DRE PSA and the post/pre-DRE geometric ratio as a coefficient of variation.

RESULTS: Thirty minutes after DRE, the increase in total, calculated complexed, and free PSA and the percent free PSA ratio was statistically significant. The difference between post-DRE and pre-DRE values of serum total, calculated complexed, and free PSA and percent free PSA was +0.9, +0.3, and +0.6 ng/mL and + 10%, respectively. The post/pre-DRE ratio of serum total, calculated complexed, and free PSA and percent free PSA was x1.5, x1.22, and x2.5 and x1.7, respectively. Patient age, DRE findings, prostate volume, prostate histologic features, and the initial value of total PSA had no statistically significant effect on the change of percent free PSA after DRE. The initial percent free PSA had a statistically significant effect on the change of percent free PSA after DRE. The values of calculated complexed PSA before and after DRE were significantly higher in patients with a prostate volume greater than 40 cc, prostate cancer on biopsy, and an initial free/total PSA ratio greater than 18%.

CONCLUSIONS: In this study, DRE significantly increased total, calculated complexed, and free PSA and percent free PSA. The increase of PSA immediately after DRE was mainly due to the increase of free PSA. The change of percent free PSA after DRE depended on the initial percentage of free PSA. Complexed PSA seems less sensitive to DRE, and its changes after DRE were clinically modest. Free PSA measurement should be done before DRE and percent free PSA should be used only when the venipuncture is done before the DRE.