DNA Samples Viable Up to 10 Days after Rape

A technical report recently made available by the NIJ, through the National Criminal Justice Reference Service, provides new research on the length of time that DNA samples may remain viable after rape.

From the Abstract of "Post-Coital DNA Recovery Study" by Patricia Speck and Jack Ballantyne:

Forensic evidence containing deoxyribonucleic acid (DNA) obtained from victims of rape contributes to elimination of suspects, conviction of others, and exoneration of those arrested and charged for rape. Using state-of-the-art laboratory methods in the 1970s, forensic science scholars found limited probative value from post-rape vaginal samples after 72 hours. Thus, time limitations of 72 hours for evidence collection remained the recommendation for most jurisdictions responding to victims reporting rape.
The Post-Coital DNA Recovery study is largest in vivo study to measure DNA detection and recovery using Y-STR methods, which specifically examine the Y chromosome. The research attempted to answer two research questions: What is the period for DNA recovery in proxy couples from the cervix and posterior fornix (a recess in the upper vagina) using Y–STR laboratory methods? And what are the common physiological conditions that may influence DNA recovery in proxy post-coital couples?
The authors successfully demonstrated the ability to obtain full and probative partial DNA profiles from enhanced Y-STR methods from the cervix, posterior fornix, and the combined samples of the cervix and posterior fornix samples collected up to nine days after intercourse in 78.8 percent of participating couples. After 10 days of abstinence, there was recovery in 67.7 percent of participating couples.
The analysis of the study data revealed that the odds of DNA recovery is significantly lower with the standard Y-STR methods when menses is reported, and when hormonal birth control is used. Data demonstrated the lowest recovery (but not absence) of DNA using the standard Y-STR method occurred when both menses and hormonal birth control are present.
This research indicates that failure to collect samples from victims with an extended post-coital interval may result in the potential loss of probative evidence that could be crucial to the investigation and prosecution of sexual crimes.
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