Biological Evidence – Key Exam Notes

Biological Evidence Overview

  • Links individuals to people, objects, locations; can associate / dissociate suspects

  • Relevant in crimes against persons ((homicide, assault, rape)), property crimes, MVAs, terrorism, mass-fatalities

  • Transfer occurs as direct or secondary contact

  • Subject to deterioration and may carry pathogens; strict safeguards required

Major Biological Evidence Types

  • Blood

  • Semen / seminal stains

  • Saliva & vaginal secretions

  • Urine & fecal material

  • Hair

  • Bone, tissue, organs

Fundamental Collection Principles

  • Wear fresh gloves; avoid talking, coughing, touching face

  • Collect maximum sample from single source; prevent mixing samples

  • Prefer submitting whole item; if not, cut out stain or swab with distilled water

  • Always take an “unstained control” from adjacent area

  • Air-dry ASAP; no heat, sunlight, or fans

  • Package each item separately in clean paper; seal, label, biohazard mark

Blood & Bloodstain Pattern Analysis (BPA)

  • Blood: vital fluid with RBCs; shed in accidents or crimes

  • Bloodstain pattern = geometric image formed when blood contacts a surface

  • BPA can indicate:
    • Positions/movement of victim & assailant
    • Weapon type; number of blows/shots
    • Event sequence; corroborate or refute statements

  • Main pattern categories: Projected, Passive, Transfer

Semen Evidence

  • Document with notes/photos/sketches

  • Liquid semen: transfer via syringe/pipette → labeled tube; keep refrigerated or absorb onto cotton

  • Stains on movables: collect item whole; air-dry first

  • Large cuttable objects: excise stained area; druggist fold; label

  • Non-absorbent immovables: scrape into druggist fold; clean tool between uses

  • Sexual-assault kits obtained in clinical setting; submit promptly

Other Biological Samples

  • Urine/Saliva (liquid): into sterile container, seal, refrigerate

  • Urine/Saliva (stain): scrape/cut; package in paper/druggist fold

  • Tissue/Bone: photo & sketch; pick up with clean forceps/gloved hand; freeze (except metal/glass substrates kept at room temp)

  • Hair: collect with forceps; separate groups; if mixed with fluids, dry before packaging

Packaging & Storage Guidelines

  • Use paper bags/envelopes large enough for airflow; never plastic/airtight for wet or semi-dry items

  • Refrigerate liquid blood; freeze dried-stain items (except metal/glass)

  • Secure sharp metal/glass in cardboard box; do not freeze

  • Tape seal, initial, date every container; avoid vehicle trunk heat during transport

Reference (Known) Samples

  • Obtain 5–7 cc5\text{–}7\,\text{cc} EDTA blood from victim & suspect

  • If blood unobtainable: collect two cheek swabs (one per cheek); dry quickly in cool air; package in labeled paper

Common Do’s and Don’ts

Do:

  • Air-dry evidence away from heat/fans

  • Refrigerate liquid blood; store others at room temp/low humidity

  • Submit entire stained items where possible; include police report
    Don’t:

  • Package wet items in plastic or airtight containers

  • Mix separate stains; freeze liquid blood; expose samples to sunlight or trunk heat

  • Remove stains from substrates unnecessarily

Submission & Documentation

  • Rapidly forward evidence to crime lab; include full documentation

  • Change gloves/tools between items; rinse tools with distilled water ×2\times 2 before reuse

  • Preserve stain patterns during drying; prevent cross-contamination