Reliability of the Prostate Acid Phosphatase (PAP) Test

Neha Vapiwala MD
Last Modified: March 27, 2007

Dear OncoLink "Ask the Experts,"
Reliability of the Prostate Acid Phosphatase (PAP) Test

My question is related to an elevated prostate acid phosphatase (PAP) of 22 ng/ml (normal = 0 - 5), and whether metastatic disease is a serious possibility since the capsule of the prostate was intact and the adenocarcinoma was well differentiated?

How reliable is an elevated PAP? Is an elevated PAP incompatible with a post-operative PSA value of < 0.1?

Thank you for whatever information you may provide.
J.A.D.P., MD

Neha Vapiwala MD, Senior Editor for Oncolink, responds:

Dear Colleague,
Thank you for your interest and question.

Acid phosphatases are a group of enzymes, which are proteins found in living organisms that are critical for various chemical reactions. In the 1930's, it was discovered that these acid phosphatases were found in high concentrations within the prostate gland. Furthermore, there were found to be responsive to androgen (male hormone) regulation. Although present in the highest concentrations in the prostate, acid phosphatases can also be found in various other tissues in the body as well as in various isoenzymatic forms. Different assays that can accurately measure acid phosphatase levels have been attempted in order to be more prostate cancer-specific. Thus, the measurement of prostatic acid phosphatase (PAP) was developed. Unfortunately, although the ability to detect very low levels of PAP exists, i.e. high sensitivity), the specificity of this test as a reliable marker of prostate cancer is still quite limited.

Other inherent problems with prostatic acid phosphatase measurements are:

PAP levels fluctuate greatly over the course of just 24 hours, whereas serum varies little over weeks to months (and sometimes years)
PAP is less stable than PSA, causing increased variability in the lab reading
False elevations of PAP after prostate manipulation are notable
Sensitivity of PAP is much less than PSA, causing more false negatives.

Based on the many documented inconsistencies with PAP, it has no significant role at present in the monitoring of patients after initial therapy. PSA is much better in that regard. Initial PAP readings maybe helpful in estimating risk of extracapsular extension: the higher the PAP, the higher the risk for having cancer outside the prostate at the time of presentation. However, this should not be the only screening test obtained since PSA has a much higher prognostic value when used in combination with digital rectal examination.