Biological Evidence Collection Protocols

No biological sample should be collected before the scene has been assessed and the evidence documented in its original state. This principle is non-negotiable: once an item is disturbed or collected, its original spatial context is lost. Documentation before collection means photography, sketching, and note-taking that capture what is present, where it is located, and its relationship to surrounding items.

The photographic record must include an establishing shot that shows the evidence in its surroundings, a mid-range shot that shows the evidence relative to adjacent items, and a close-up shot taken perpendicular to the surface with a scale marker. For bloodstains, the pattern must be documented before any stains are swabbed because swabbing destroys pattern morphology. Bloodstain pattern analysis depends on the complete, undisturbed pattern, and collection must be planned to preserve the pattern record.

A written scene log should record the time of arrival, environmental conditions (temperature, humidity, whether the scene is indoors or outdoors, any active degradation risks such as rain or direct sunlight), and all personnel present. Each item of biological evidence is assigned a unique item number before collection begins, and this number links the scene photographs, the collection log, the packaging label, and the laboratory submission form.

Biological evidence scenes present two simultaneous PPE requirements: protecting the evidence from the collector, and protecting the collector from the evidence. Modern PCR-based DNA profiling is sensitive enough to detect a single cell. This means that a collector who sneezes near a stain, touches a surface without gloves, or uses the same swab packaging between items can introduce foreign DNA that becomes part of the analytical result.

Gloves must be changed between each item of evidence collected, not merely between major scene areas. Gloves that have touched one stain carry DNA from that stain and will transfer it to the next item handled. A fresh pair per item is the minimum standard. Some laboratories also require collectors to have their own DNA profiles on file for elimination purposes, so that any inadvertent contamination can be identified and excluded at the analysis stage.

Secondary transfer is a separate concern: DNA from person A transfers to surface B, person B then contacts surface B and picks up A's DNA, and B's contact with surface C deposits A's DNA on C. A profile found on an item does not necessarily mean that person had direct contact with that item. This is not a collection error; it is an interpretive consideration. But collection errors can create artificial secondary transfer scenarios in the laboratory if proper glove-change discipline is not maintained.

The choice of collection technique depends on the substrate (the surface the stain is on), whether the stain is wet or dry, and the likely evidence type. No single technique is universally optimal. The four primary techniques are swabbing, cutting, scraping, and liquid collection.

Swabbing is used for stains on non-cuttable surfaces: walls, floors, vehicles, skin, and irregular objects. For dried stains, the double-swab technique is the current validated standard. The first swab is moistened with sterile distilled water (never saline, which inhibits PCR) and applied with firm circular pressure over the stain area. The second dry swab immediately follows. Both swabs air-dry before packaging. Swabs must never be sealed wet: a wet swab sealed in a container creates conditions for mould and bacterial growth that will degrade DNA within hours.

Cutting is used when the stain is on a cuttable substrate: clothing, upholstery, carpet, bedding, or similar textiles. A portion of the stained material is cut out using clean scissors or a scalpel, taking care to include the full stain plus a margin and to cut a control section from an unstained adjacent area. Cutting is preferred over swabbing for textiles because more cellular material is retained within fabric fibres than can be lifted by a swab surface. The cut section is air-dried if wet, then packaged in paper.

Scraping is used for dried, flaking biological material, typically dried blood or seminal crust on hard non-porous surfaces where swabbing would smear the stain across the surface rather than lifting it. A clean scalpel or scraping tool is used to dislodge the dried material onto clean paper, which is then folded and packaged. Scraping is also used for paint that has blood dried within it, or for biological material on glass.

Alternate light sources exploit the optical properties of biological fluids to make them visible under conditions where they are invisible under white light. Semen fluoresces strongly at excitation wavelengths of approximately 420 to 470 nm, producing a blue-white emission visible through an orange or yellow barrier filter. Saliva and urine produce weaker fluorescence at similar wavelengths. Some bloodstains absorb light at specific wavelengths and appear darker than the surrounding background rather than fluorescing.

Common ALS devices used in forensic scenes include the Polilight (ROFIN), CrimeScope, and BlueStar Forensic for bloodstain enhancement. Wavelength selection matters: a single wavelength is rarely optimal for all fluid types. Many scene investigation kits carry ALS units with tunable wavelengths from approximately 350 nm (UV) through to 700 nm (red), with wavelength-specific barrier filters for each range.

ALS examination has important limitations. Fluorescence is not specific to biological material. Petroleum products, optical brighteners in clothing and paper, certain dyes, and some adhesive residues also fluoresce at forensically relevant wavelengths. Every ALS-positive area must be presumptively tested before being described as biological in nature. ALS identifies where to look; confirmatory tests establish what the material is.

The sequence at a scene is: ALS examination first under controlled lighting conditions, photograph every fluorescing area with the ALS illumination and filter in place, mark each area with an exhibit number, then proceed to presumptive testing and collection. This sequence ensures the spatial pattern of biological material is documented before any item is disturbed.

Every item of biological evidence must be packaged in breathable paper: paper bags, coin envelopes, or folded paper wraps depending on the item size. Plastic bags are prohibited for biological evidence because they trap moisture. The single exception is liquid samples in sealed collection tubes. Once packaged, the package is sealed with tamper-evident tape and the seal is initialled by the collector.

The label applied to each package must include: the case reference number, the item number as assigned in the scene log, a description of the item, the collection location (specific, not 'crime scene'), the name and signature of the collector, and the date and time of collection. These fields correspond to the minimum information required by most national forensic standards and by the chain of custody requirements of evidence law in the jurisdictions where the evidence will be used.

Chain of custody is the documented record of every transfer of the item from that point forward. Each transfer is recorded on a continuity form or in an evidence management system: who transferred the item, to whom, at what time, and for what purpose. In the UK, the Criminal Procedure and Investigations Act 1996 and the associated Code of Practice require that the chain be maintained and disclosed to the defence. Under the Bharatiya Sakshya Adhiniyam 2023 in India, evidence must be accounted for continuously from seizure to production before the court. In the US, Federal Rule of Evidence 901 requires authentication of physical evidence, which in practice depends on an unbroken chain of custody record.

DNA in biological evidence degrades through four primary pathways: hydrolysis (water-mediated cleavage of the phosphodiester backbone), oxidation (damage from reactive oxygen species and UV radiation), microbial activity (nucleases produced by bacteria and fungi that colonise wet biological material), and mechanical fragmentation (physical shearing from freeze-thaw cycles or rough handling). The rate of degradation is determined by environmental conditions, and controlling those conditions is the practical goal of preservation protocols.

Outdoor scenes present the highest degradation risk. A bloodstain on tarmac in direct summer sunlight can become unprofilable within hours through combined UV exposure, hydrolysis from dew, and microbial colonisation. Indoor scenes in climate-controlled environments may preserve DNA for months or years. Understanding the environmental context is part of prioritising which items to collect first: the most environmentally exposed items should be collected before items in protected locations.

Long-term storage of biological evidence for cases awaiting trial or potential review requires freezing at minus 20 degrees Celsius or colder. Most reference samples and extracted DNA are stored at minus 20 degrees Celsius, while original evidence items are typically refrigerated if they are expected to be needed within months, or frozen at minus 80 degrees Celsius for archival storage. Evidence management policies for long-term cases must specify the storage conditions and require regular checks that refrigeration has not failed.