PSA
PSA is a glycoprotein produced by prostatic epithelial cells that can be elevated by cancer, but also by BPH, prostate infections, or ejaculation, causing results that are false positives. About 12 in 100 men have a PSA screening test above the level of 4 ng/mL, but only 30% of these men will have prostate cancer on biopsy (the positive predictive value). Overall, using a PSA level of 4 ng/mL to define abnormal detects only 21% of prostate cancers (sensitivity), but 51% of aggressive cancers, based on the microscopic appearance of cancer cells. The associated specificity is 91%, which is the proportion of men without prostate cancer with a normal test. Numerous modifications of PSA have been proposed to increase its accuracy, including measuring changes over time (velocity), the proportion of PSA that is not bound to protein (free), and the PSA density (based on the prostate volume) as well as adjusting the cut-off for abnormal based on the patient's age or race. However, none of these strategies has been shown to improve outcomes, and guidelines recommend against them.
DRE
The DRE explores for palpable abnormalities such as nodules, induration, or asymmetry in the peripheral posterior and lateral areas of the prostate gland closest to the examining finger; the DRE is unable to detect cancers in the anterior and central areas of the gland. The sensitivity of DRE is 59%, and the specificity is 94%. An estimated 28% of men with an abnormal DRE will have prostate cancer on biopsy (positive predictive value). However, the majority of prostate cancers detected by DRE have already spread beyond the prostate gland, making them more difficult to cure. Furthermore, the DRE is not very reproducible (low kappa score)—even urologists have problems agreeing with each other whether the DRE is abnormal.
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