Acid phosphatase testing for rape

Medical Laboratory Observer, June, 2005 by Frank Cruz, III, Acid Phosphatase

Q The ED physician in charge of our hospital's rape-crisis program would like our lab to use prostatic acid phosphatase assays to assist physicians in supporting rape claims. Currently, our laboratory performs acid phosphatase (total) testing on saline from vaginal swabs. Neither manufacturer of our two main chemistry analyzers offers a prostate-specific acid phosphatase assay. Any advice on our options for test assays that would allow us to provide the ED physicians with a STAT result to support a rape? Also, any recommendations or references for a cut-off value for total acid phosphatase on vaginal testing?

A After an alleged sexual assault, sperm and prostatic acid phosphatase in vaginal fluid are considered important physical evidence that often can be used in court if a rape trial occurs. (1) According to current recommendations, analyses should be performed in forensic laboratories; providers should use evidence-collection kits obtained through state distribution programs or directly from kit manufacturers; and providers should pay careful attention to guidelines for specimen collection. (2)

The presence of semen can be confirmed by the finding of spermatozoa, which can remain in the vagina for up to 72 hours after intercourse. (1,3) Although acid phosphatase is present in high levels in seminal samples, it is considered only a strong indication of the presence of semen because it can also appear in vaginal secretions; however, seminal plasma glycoprotein p30, an antigen specific to the prostate, is widely regarded as conclusive proof of the presence of semen and indicates ejaculation during the previous 48 hours. Currently, seminal plasma glycoprotein p30 measurements are the standard of care in forensics laboratories. (2)

Reference ranges for prostatic acid phosphatase in post-rape situations have been largely the result of studies that took place in the 1970s and early 1980s. Values of prostatic acid phosphatase activity greater than 50 Sigma units/cc, and particularly those greater than 138, were found to correlate with intercourse within the preceding 24 hours. Values greater than 20, but less than 50, were found to correlate with intercourse within the preceding 48 hours. The presence or absence of sperm was found to be less sensitive and to correlate poorly with the time since intercourse. (1) In a series of post-mortem examinations using ELISA methods, a level of prostatic acid phosphatase greater than 100 ng/mL was considered positive for sexual intercourse. (4)

References

1. Ricci LR, Hoffman SA. Prostatic acid phosphatase and sperm in the post-coital vagina. Ann Emerg Med. 1982;11:530-534.

2. Young WW, Bracken AC, Goddard MA, Matheson S. Sexual assault: review of a national model protocol for forensic and medical evaluation. New Hampshire Sexual Assault Medical Examination Protocol Project Committee. Obstet Gynecol. 1992;80:878-883.

3. Hampton HL. Care of the woman who has been raped. N Engl J Med. 1995;332:234-237.

4. Collins KA, Bennett AT. Persistence of spermatozoa and prostatic acid phosphatase in specimens from deceased individuals during varied postmortem intervals. Am J Forensic Med Pathol. 2001;22:228-232.

--Frank Cruz, III, MD

Department of Pathology

Oregon Health and Science University

Portland, OR

Edited by Daniel M. Baer, MD

MLO's "Tips from the Clinical Experts" provides practical, up-to-date solutions to readers' technical and clinical issues from a panel of experts in various fields. Readers may send questions to Dan Baer by e-mail at tips@mlo-online.com.