Identification of female cells in postcoital penile swabs using fluorescence in situ hybridization - Application in sexual assault

Traditionally, the finding of semen, that is, spermatozoa and acid phosphat ase, in cervicovaginal specimens has been considered the laboratory evidenc e needed to prove recent sexual contact. Recent research with fluorescence in situ hybridization (FISH) has shown that in the absence of semen, male e pithelial and inflammatory cells can be found within the female genital tra ct. A striking paucity of literature exists pertaining to the examination o f the penis of an alleged assailant for potential evidence indicative of se xual assault. The current study uses FISH to analyze postcoital swabs of th e penis for such laboratory evidence. A male and female volunteer couple co nsented to participate in this study. Following coitus, the male partner pr esented to one of the investigators for penile swabbing. Swabs were taken a t varying postcoital intervals (1-24 hours) subsequent to 10 coital episode s. The male participant was instructed not to shower following coitus, but to otherwise go about daily activities until specimen collection. To obtain each sample, 4 sterile cotton-tipped applicators were slightly moistened i n sterile saline and swabbed along the length of the penile shaft and aroun d the base of the penis. From the swabs, 3 air-dried slides were prepared, coded, and blinded. As controls, swabs were taken from the buccal surfaces of both volunteers. Multicolor FISH was performed using dual X- and Y-chrom osome probes, and slides were counterstained with 4'-6-diamidino-2-phenylin dole (DAPI). Cells were easily visualized under a fluorescent microscope, b ut only cells with 2 nonoverlapping fluorescent signals were counted. Fluor escence in situ hybridization is highly sensitive and specific, and the dua l probes easily distinguished between male and female cells. Female cells w ere identified on smears from every penile swab over the entire 1- to 24-ho ur postcoital interval. The FISH technique, previously successful in identi fying male cells within the female genital tract, may also be employed on p enile swabs. Once the presence of female cells is confirmed by FISH, the id entity of the female can be confirmed by DNA analysis. Potentially, with su ch current molecular analyses, both the assailant and the victim can be pos itively identified.