Prostatic acid phosphatase and sperm in the post-coital vagina

Sperm and prostatic acid phosphatase identified in vaginal fluid after an alleged sexual assault constitute important physical evidence useful in courts of law during rape trials. The purpose of this study was to define a normal and abnormal range for acid phosphatase in the post-coital vagina for our emergency department and to compare the sensitivity of acid phosphatase versus sperm in determining recent coitus. Vaginal swabs obtained from 90 patients were analyzed for acid phosphatase activity by the Sigma p-nitrophenyl phosphate technique. Swabs were stored in a bovine albumin preservative broth. Pap smears for sperm identification were also performed. Acid phosphatase values greater than 50, especially those greater than 138 Sigma units/cc, correlated with intercourse within the preceding 24 hours (P .005). Values greater than 20 but less than 50 correlated with intercourse within 48 hours (P .005). The presence or absence of sperm was found to be less sensitive, and correlated poorly with the time since intercourse [of 12 cases less than or equal to 24 hours, 11 were acid phosphatase-positive (91.7%) while seven were sperm-positive (58.4%)].