Forensic Evidence Identification, Collection, and Preservation
Categories of evidence
Trace (fibers, hair, debris, glass, soil), biological (blood, saliva, semen, vaginal fluid, urine), impression and pattern evidence, digital/photographic documentation, and clothing. Deoxyribonucleic acid (DNA) is the most commonly adjudicated biological evidence, but trace evidence can corroborate or refute an account.
Collection principles
Collect only what is indicated, but when in doubt and with consent, collect and document. Swab visible stains and body sites per protocol (oral, vaginal, cervical, anal, skin), label each swab with site, date, time, and collector initials, air-dry thoroughly, and package in paper. Plastic traps moisture and promotes degradation and mold.
Avoid cross-contamination: change gloves between sites, do not talk or cough over open samples, use clean paper on the floor, and package each item separately. Obtain a buccal reference sample per jurisdictional protocol and with consent. Urine for drug-facilitated sexual assault should be collected as early as possible and preferably as a first void; some drugs clear quickly, so delay reduces yield.
Time frames (commonly 72–120 hours for biological collection, longer for DNA on some surfaces) are program- and jurisdiction-specific, not physiological absolutes; a patient outside the window may still have evidence value.
Integrity
Maintain chain of custody from the moment of collection: every transfer documented with name, date, time, and signature. Store per protocol, and release only to authorized persons.
Traps
- Plastic bags for damp items.
- Swabbing after the patient has washed, or failing to note that washing occurred.
- Treating the collection window as an absolute bar.
- Collecting without consent or over objection.
Rule of thumb: Dry it, paper it, label it, seal it, sign for every hand it touches.