Collection Techniques for Biological Evidence

Why Biological Evidence Demands Special Handling

Biological evidence — bloodstains, saliva, hair, and other body fluids — contains DNA and other trace materials that can identify suspects, exonerate the innocent, and establish timelines. It is also fragile. Moisture promotes bacterial growth that degrades DNA. Cross-contamination from a technician's own skin cells or breath can introduce foreign profiles. Improper packaging can invalidate results and compromise the chain of custody, potentially collapsing a prosecution. Your first duty at the scene is to preserve the integrity of what you collect.

Personal Protective Equipment and Safety First

Before touching anything, don full PPE: nitrile gloves (double-glove if working in a heavily contaminated area), a Tyvek suit, a face mask, and safety glasses. Assume every bloodstain is a potential biohazard. Change gloves between samples to prevent cross-contamination. Never eat, drink, or touch your face on scene. Dispose of used swabs and contaminated gloves in a biohazard waste container, not general trash. If a glove tears, stop, remove both gloves, wash hands, and re-glove before continuing.

Wet-Swab / Dry-Swab Technique for Bloodstains

The double-swab technique (also called the wet-then-dry method) is the accepted standard for collecting dried bloodstains from non-porous and semi-porous surfaces.

Step 1 — Wet swab: Moisten a sterile cotton swab with a small amount of sterile distilled water. Do not soak it; the tip should be damp, not dripping. Swab the stain with gentle circular pressure from the outer edge toward the center. This rehydrates and lifts the dried biological material.

Step 2 — Dry swab: Immediately follow with a second dry cotton swab over the same area. The dry swab absorbs residual moisture and picks up any material the wet swab left behind. The dry swab also acts as a reference for the wet swab's condition.

Step 3 — Air-dry: Both swabs must air-dry completely at room temperature before packaging. Never use heat. Wet swabs sealed in airtight containers grow mold and bacteria that destroy DNA within hours.

For fabric evidence (a bloodstained piece of clothing, for example), cutting is preferred over swabbing when possible. Use clean scissors to excise the stained portion — cut at least a small border of unstained fabric around the stain. Place the cutting in a paper envelope or paper bag; allow it to air-dry if still moist.

Collecting Hair and Fiber Evidence

Hair and fiber are trace evidence and must be collected before biological swabs to avoid disturbing their position. Use clean forceps (not your fingers) to pick up individual hairs. For loose surface hairs or fibers over a large area, a tape lift is more efficient.

Tape-lift technique: Press a length of clear tape firmly over the surface, then lift it in one smooth motion. Mount the tape immediately, sticky-side-down, onto a sheet of acetate backing the same size. Label the acetate on the non-sticky margin. Each tape lift is one item of evidence requiring its own label and submission form. Take reference (control) tape lifts from unaffected areas nearby so the laboratory can distinguish scene fibers from background contamination.

Forceps must be cleaned with a lint-free wipe and 70% isopropyl alcohol between each hair collected, or a fresh set of forceps used, to prevent cross-contamination between items.

Packaging Standards for Biological Evidence

The single most important packaging rule: biological evidence goes in paper, never plastic. Paper allows moisture vapor to escape. Plastic traps it, promoting microbial growth and DNA degradation.

• Place dried swabs and cuttings in paper envelopes or paper bags.
• Include a desiccant packet inside the container to absorb residual moisture during storage and transport.
• Seal the container, then run evidence tape across the seal. Initial (sign) across the tape so any tampering is visible.
• Label every container with: case number, item number, brief description of contents, date and time of collection, location collected, and your name or badge number. Missing any field is a chain-of-custody failure.

Chain of Custody: The Paper Trail That Makes Evidence Admissible

Chain of custody is the unbroken, documented record of who collected, handled, transferred, and stored an item of evidence. Every time an item changes hands, an entry must be made on the chain-of-custody form: person releasing, person receiving, date, time, and purpose. Gaps in the chain give defense attorneys grounds to challenge admissibility. Complete your entry at the moment of collection, not later from memory.

Common Errors and How to Avoid Them

• Using plastic bags: Degrades DNA. Use paper.
• Wet packaging: Always air-dry before sealing.
• Gloves not changed between samples: Transfers DNA from one item to another.
• Incomplete labels: Chain of custody is broken.
• Swabbing too aggressively: Can mix adjacent stains. Work from the outer edge inward.
• Skipping reference samples: The lab needs an unaffected control to interpret results.

Follow these principles consistently and your packaged evidence will pass laboratory intake, support a valid DNA profile, and hold up in court.