Y‐STR Profiling in Extended Interval (≥3 days) Postcoital Cervicovaginal Samples*

Abstract:  Depending upon specific situations, some victims of sexual assault provide vaginal samples more than 36–48 h after the incident. We have tested the ability of commercial and in‐house Y‐STR systems to provide DNA profiles from extended interval (≥3 days) postcoital samples. The commercial Y‐STR systems tested included the AmpFℓSTR® Yfiler™ (Applied Biosystems), PowerPlex® Y (Promega) and Y‐PLEX™ 12 (Reliagene) products whereas the in‐house systems comprised Multiplex I (MPI) and Multiplex B (MPB). Three donor couples were recruited for the study. Postcoital cervicovaginal swabs (x2) were recovered by each of the three females at specified intervals after sexual intercourse (3–7 days). Each time point sample was collected after a separate act of sexual intercourse and was preceded by a 7‐day abstention period. As a negative control, a precoital swab was also recovered prior to coitus for each sampling and only data from postcoital samples that demonstrated a lack of male DNA in the associated precoital sample was used. A number of DNA profile enhancement strategies were employed including sampling by cervical brushing, nondifferential DNA extraction methodology, and post‐PCR purification. Full Y‐STR profiles from cervicovaginal samples recovered 3–4 days after intercourse were routinely obtained. Profiles were also obtainable 5–6 days postcoitus although by this stage partial profiles rather than full profiles were a more likely outcome. The DNA profiles from the sperm fraction of a differential lysis were superior to that obtained when a nondifferential method was employed in that the allelic signal intensities were generally higher and more balanced and exhibited less baseline noise. The incorporation of a simple post‐PCR purification process significantly increased the ability to obtain Y‐STR profiles, particularly from 5‐ to 6‐day postcoital samples. Remarkably an 8 locus Y‐STR profile was obtained from a 7‐day postcoital sample, which is approaching the reported time limit for sperm detection in the cervix.