Before the existence of the PSA test, the prostatic acid phosphatase or PAP test was very important because it was the only test we had that could tell whether a patient might have metastatic disease, even if there were no signs of bone mets on a bone scan. The PAP test was used as a tumor marker after Gutman noted, in 1938, that elevated levels of this enzyme are found in patients with metastatic prostate cancer.
The PAP test, like the PSA test, is a simple blood test. It is used to measure the amount of an enzyme called prostatic acid phosphatase (PAP). PAP is an enzyme that is found in in the prostate gland and semen of men. If the prostate isn’t working properly, then PAP is released into the blood. In this way, the test has many similarities to the PSA test. However, the PAP test isn’t generally as useful as the PSA test, and so most of the time it has been replaced by the PSA test.
The one place where, sometimes, the PAP test can still be valuable is in patients with an elevated PSA but no sign of bone metastases. If the PAP level is also elevated in such patients, then there is a strong possibility that the patient already has metastatic disease, and so (for example) pelvic radiation in an attempt to prevent progression to metastatic disease would probably not be useful.
THE "NEW" PROSTATE CANCER INFOLINK 