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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Crime Scene / Forensic Technician › Bloodstain & Trace Evidence Basics › Lesson 7
AGR-CSI53-05 · Lesson 7 · 15 clock hours · lab

Biological Evidence Collection: Swabbing, Cutting, and Packaging

Objective. Students will collect wet and dried simulated bloodstain samples using double-swab, single-swab, and cutting methods, then package and label each sample correctly for laboratory submission

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Biological Evidence Collection Matters

Bloodstain and biological trace evidence can identify victims and suspects, establish timelines, and link people to scenes. That power is only realized if the evidence reaches the laboratory in a condition that allows accurate DNA analysis. Poor collection or packaging decisions — especially sealing wet samples in plastic — can destroy that evidence before it is ever tested. Every step you take at the scene, from the first photograph to the final seal of a paper bag, is part of the legal and scientific chain that will be scrutinized in court.

Safety First

All biological evidence must be treated as potentially infectious material. Before you touch any stain or biological sample, put on nitrile examination gloves. Change gloves between each item to prevent cross-contamination. If aerosols are possible (scraping dried stain), wear a surgical or N95 mask and safety glasses. Keep scalpel blades in a puncture-resistant container when not in use and dispose of used blades immediately in a sharps container — never re-cap by hand. Work surfaces should be covered with clean bench paper that is discarded between items. Your PPE is not optional; biological hazards at crime scenes fall under OSHA Bloodborne Pathogen standards.

The Double-Swab Technique

The double-swab technique is the standard method for collecting a dried stain from a non-porous surface such as tile, glass, or painted wall. It was developed to maximize the recovery of cellular material from hard surfaces.

Step 1 — Photograph first. Before touching anything, photograph the stain with and without a scale marker. Document location, orientation, and size.

Step 2 — Wet swab. Moisten a sterile cotton swab with a small amount of sterile distilled water. Roll the wet swab firmly over the entire stain using a circular motion, rotating the swab as you work so that fresh cotton contacts the surface throughout. The moisture rehydrates dried cells and lifts them onto the swab.

Step 3 — Dry swab. Immediately follow with a second sterile dry swab over the same area. The dry swab absorbs excess moisture and picks up additional cellular material left behind.

Step 4 — Air-dry. Both swabs must be air-dried completely before packaging — never seal a wet swab. Lay them on clean bench paper in a secure area. Moisture trapped inside a package promotes mold and bacterial growth, which degrades DNA.

Step 5 — Package. Place each swab in a separate paper coin envelope. Label the envelope with the case number, item number, date, time, location of collection, and your initials. Paper allows vapor to escape; plastic traps moisture and ruins the sample.

The Cutting Method

When the stain is on an absorbent material — fabric, carpet, upholstery — the best approach is to physically remove the stained portion.

Scalpel safety: Attach and remove blades using a hemostat or blade-removal tool, never your fingers. Cut away from your body. Dispose of blades in a sharps container immediately after use.

1. Photograph the item and stain with scale before cutting.

2. Using sterile forceps and a sterile scalpel, excise a representative portion of the stain — include some of the margin between stained and unstained fabric to help the laboratory assess the sample. Cut a control swatch from an unstained area of the same fabric and package it separately.

3. Place the cut swatch into a druggist fold (pharmaceutical fold) made from clean white paper: fold the paper over the swatch in thirds, then fold the ends in so the swatch cannot fall out.

4. Place the druggist fold inside a paper bag. Seal the bag with evidence tape and initial across the tape seal so any tampering is visible.

5. Complete a chain-of-custody tag and attach it securely to the outside of the bag.

Whole-Object Submission

Sometimes the stained item itself must go to the laboratory — for example, a ceramic tile. Non-porous items may still carry touch DNA or liquid residue and must be submitted intact.

1. Photograph and sketch the item with measurements.

2. Allow any wet blood to air-dry before packaging.

3. Place a desiccant packet inside a paper bag, then place the item inside. Desiccant absorbs ambient humidity during storage and transit.

4. Seal with evidence tape; initial the seal.

5. Complete the property/evidence submission form in full. Required fields typically include case number, submitting officer/technician, date and time of collection, item description, location found, and chain-of-custody signatures. No field should be left blank — write "N/A" if a field genuinely does not apply.

Packaging Rules: Paper, Not Plastic

The single most important packaging rule for biological evidence is: use paper, never plastic. Paper is breathable and allows residual moisture to escape. Plastic seals in moisture, which accelerates bacterial and fungal growth and degrades DNA. This rule applies to swabs, fabric cuttings, and paper substrates alike.

Chain of Custody

Chain of custody is the documented, unbroken record of who possessed an item from collection through courtroom presentation. Every person who handles the evidence must sign and date the chain-of-custody tag. Any gap or illegible entry can allow a defense attorney to challenge the integrity of the evidence. Treat the tag as part of the evidence itself.

Photograph Before and After

Always photograph the stain before collection and the surface after collection. The "after" photograph documents what was taken and confirms the area was sampled. Both sets of images become part of the case file.

Key terms8

Double-swab techniqueA collection method using a wet swab followed immediately by a dry swab over the same dried stain on a non-porous surface, maximizing recovery of cellular material.
Druggist foldA method of folding paper around a small item — such as a fabric cutting — in overlapping thirds so the contents are secured and cannot spill out; used to package trace and biological evidence.
Chain of custodyThe continuous, documented record of every person who collected, handled, transferred, or examined a piece of evidence from the crime scene to the courtroom.
Control sampleA sample collected from an area near a stain that shows no visible contamination, used by the laboratory to detect background DNA or contamination in the substrate.
DesiccantA moisture-absorbing material (such as silica gel) placed inside an evidence package to reduce humidity and help prevent mold and bacterial growth during storage.
Air-dryingAllowing wet swabs or other biological samples to dry completely at ambient temperature before final packaging, preventing moisture buildup that degrades DNA.
Bloodborne Pathogen StandardAn OSHA regulation requiring employers and workers to use PPE and safe work practices when there is a risk of exposure to human blood or other potentially infectious materials.
Sharps containerA puncture-resistant, closeable container used for safe disposal of scalpel blades, needles, and other sharp objects at a scene or laboratory to prevent accidental injury.
In class: Demonstrate and practice the double-swab technique on a dried simulated stain on a non-porous surface: wet swab first, then dry swab; students photograph the stain before and after collection, air-dry both swabs, and package in paper coin envelopes · Cutting method: using sterile scalpel and forceps, students excise a portion of a fabric swatch bearing a dried stain, package in a paper fold (druggist fold), place in a paper bag, and seal with evidence tape; complete a chain-of-custody tag for the item · Whole-object submission exercise: students prepare a mock non-porous item (ceramic tile with simulated blood) for submission—photograph, measure, sketch, seal in a paper bag with desiccant, complete a property/evidence submission form with all required fields
Materials: Sterile cotton swabs, sterile scalpel handles and blades, forceps, paper coin envelopes, paper bags, evidence tape, permanent markers, chain-of-custody tags, property/evidence submission forms, desiccant packets, simulated blood on fabric swatches and ceramic tiles, camera
Check: Instructor evaluates completed samples using a packaging checklist: all samples must be in paper (not plastic), chain-of-custody tag must be complete and unbroken, swabs must be air-dried before final packaging, and submission form must have zero blank required fields; any packaging error that could degrade DNA is an automatic fail

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Crime scene evidence collection
1. When collecting a dried bloodstain from a painted concrete wall using the double-swab technique, which swab is applied first?
Crime scene evidence collection
2. A technician collects swabs from a dried stain and immediately seals them in plastic zipper bags to protect them during transport. What is the most significant consequence of this action?
Crime scene evidence collection
3. Which packaging approach is correct for a fabric swatch with a dried bloodstain cut from a piece of clothing?
Health and safety / PPE
4. A technician is about to remove a used scalpel blade after cutting a fabric swatch. What is the safest procedure?
Crime scene evidence collection
5. Why should a technician collect a control sample from an unstained area of the same fabric when using the cutting method?
Chain of custody and documentation
6. During the whole-object submission exercise, a student leaves the 'Submitting Technician' field blank on the property/evidence submission form because the item will be delivered in person. What is the correct action?
Crime scene documentation
7. A student photographs a simulated bloodstain on a ceramic tile before collection but forgets to take an 'after' photograph. What information is lost?
Crime scene evidence collection
8. A packaging checklist used by an instructor includes an 'automatic fail' condition. Which of the following errors would most likely trigger that condition during evaluation of submitted samples?
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