Interpretation of Postmortem Vaginal Acid Phosphatase Determinations

Abstract

Identification of evidence indicative of sexual assault, including elevated levels of acid phosphatase (AP) in the vagina, is an important part of the investigation of female homicidal deaths. In this study, vaginal AP levels in 43 female homicide victims, determined using sodium thymolphthalein as the substrate, were retrospectively correlated with semiquantitative assessment of spermatozoa in vaginal smears. The results were then compared with those from a prospectively studied age-matched group of controls. AP determinations ranged from 0 to 6,000 units (U)/L in the homicide group and from 1 to 726 U/L in the control group. In both groups, the majority of the AP values (homicide, 65%; and controls 85%) were < 100 U/L. Spermatozoa were identified on vaginal smears from 30% of the homicide victims and 17% of the controls. The spermatozoa-positive cases had AP levels that ranged from 2 to 6,000 U/L. In both groups, an AP value of > or = 400 U/L was always associated with the presence of spermatozoa on the vaginal smear. Intermediate AP values (100-399 U/L) were associated with the presence of spermatozoa in 60% of the control group and 22% of the homicide group; 18% of the homicide group and 3% of the control group were positive for spermatozoa despite AP values of < 100 U/L. In the absence of spermatozoa on a vaginal smear, an AP value of > 400 U/L strongly suggests the presence of semen that is either oligo- or aspermic. A low or intermediate AP determination, however, does not exclude the presence of semen and hence a careful search for spermatozoa on the vaginal smear is indicated, regardless of the AP value obtained.