Prostate Acid Phosphatase PAP Test
The PAP test is another blood test, like PSA, that used to be used to screen for prostate cancer. It has been replaced by the PSA test, and is no longer used much. It measures acid phosphatases, a group of enzymes that are found in the highest concentration in the prostate but are also found in other tissues. Different assays have been attempted to make the test more specific to prostate cancer, but the test has never been improved much.
Problems with PAP measurements, compared to PSA, are the following:
- PAP levels fluctuate more over the course of a day; PAP is less stable than PSA.
- It is affected more by prostate manipulation, resulting in more false-positive tests.
- Sensitivity is much less than PSA; there are more false negatives.
Because of these problems, PAP is no longer used for screening. PSA with DRE has a much higher prognostic value. PAP may be helpful, however, in estimating the risk of a tumor spreading outside the gland -- the greater the PAP level, the greater the risk for having advanced disease.
