Low-Template DNA, a Bite Mark, and Fibers: Collection Priorities After a Shower
A 22-year-old woman reports a sexual assault that occurred about 36 hours ago. She showered twice and changed clothes. Her clothing was stored in a plastic bag for 8 hours before being handed to police. A condom is in a trash can, fibers are visible on a bedsheet, and a bite mark is on her shoulder. The medical examination is scheduled in 1 hour. The detective asks you to set collection priorities and warns that the condom may be the only source of the assailant's DNA. You have a sexual assault kit, sterile swabs, paper bags, and a trace evidence kit, but only one evidence drying cabinet.
1. Prioritize biological and trace evidence collection from the victim and scene, explaining the trade-offs given the shower and the 36-hour delay.
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Victim first: complete the sexual assault kit, including vulvar, vaginal, cervical, oral, anal, and skin swabs, plus fingernails and the bite-mark swabs. The shower reduces surface DNA and semen, so focus on areas less likely washed-cervix, anus, mouth-and on the bite mark, where saliva may remain. Collect the clothing and air-dry it; plastic storage for 8 hours may have promoted moisture and degradation, so document the condition. Scene: the condom is high priority for DNA; the bedsheet is high priority for fibers and biological stains. Use the drying cabinet for the wet or damp items first. The trade-off is that the condom may dry out or be contaminated if ignored, but the victim's kit is time-sensitive and medically urgent; coordinate so the kit is collected before she showers again or changes clothes.
2. How do you collect and preserve evidence from the bite mark, and what should you not do?
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Photograph the bite mark with a scale and overall orientation, then swab the area using a double-swab technique: a wet sterile swab followed by a dry swab, with gentle rotation over the bite area. Collect a reference DNA sample from the victim. Air-dry the swabs, package separately, and label the location and time. Do not scrape, cut, or excise the bite mark; do not apply powder or chemicals; do not let the victim wash the area before collection. Reasoning: saliva DNA may be present in low quantity, and mechanical or chemical treatment can remove or degrade it. If the bite mark is photographed for pattern interpretation, use consistent lighting and a scale, but treat the DNA collection as the priority.
3. Only a few fibers and a small stain are visible. What is your approach to maximize recovery and avoid loss, and how does low-template DNA affect your packaging?
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For the few fibers, use clean forceps or a tape lift rather than a vacuum, which can lose them in the filter or dilute them. Photograph the area first, then lift the fibers onto paper or a sterile container. For the small stain, swab with a minimal amount of moisture and a sterile swab; air-dry. Low-template DNA requires contamination controls: wear a mask, change gloves frequently, avoid speaking over the evidence, use sterile tools, and package each swab separately with negative controls. Use paper packaging, not plastic, to prevent moisture. Reasoning: few fibers are easily lost, and low-template DNA is easily contaminated or diluted; minimal handling and separate packaging preserve both the trace and the DNA.