DNA evidence is collected and preserved through a controlled sequence: investigators secure the scene, document it before anything moves, locate biological material with the naked eye or specialized lights, lift it with sterile swabs or by taking the whole item, air-dry it, seal it in paper packaging, and log every person who handles it from the scene to the courtroom. The point of every step is the same: keep the biological material intact and keep foreign DNA out of it.
What Counts as DNA Evidence
Any biological material a person leaves behind can carry a usable genetic profile. Blood, saliva, semen, urine, sweat, hair with roots, skin cells, bone, and teeth are all viable sources. Investigators focus on items where those materials tend to land: weapons, clothing, bedding, masks, hats, gloves, drinking cups, cigarette butts, chewing gum, envelopes, and fingernail scrapings.
The less obvious category is touch DNA, the microscopic skin cells shed just by gripping a steering wheel or pressing a light switch. Nobody has to bleed or spit on something to leave a genetic signature. Touch DNA has widened the range of useful evidence, but it complicates interpretation, because the quantities are tiny and secondary transfer is possible. Someone who shook hands with a perpetrator and then touched an object at the scene could, in theory, deposit that perpetrator’s DNA without ever being there.
Securing the Scene and Preventing Contamination
Before anyone touches potential evidence, the scene is locked down. Investigators set a perimeter with crime scene tape and post an officer to keep an entry and exit log. That log records every person who enters, their arrival and departure times, their organizational affiliation, and their reason for being there.1National Institute of Standards and Technology (NIST). Crime Scene Investigation – A Guide For Law Enforcement If someone leaves without checking out, the log officer notes an estimated departure time with an explanation. The record controls access in the moment and later proves no unauthorized person introduced contamination.
Everyone inside the perimeter wears personal protective equipment: gloves, often doubled, along with face masks, shoe covers, and sometimes full protective suits. A single hair falling from an investigator’s head or a sneeze near a blood sample can introduce foreign DNA and compromise the entire case.2United Nations Office on Drugs and Crime (UNODC). Crime Scene and Physical Evidence Awareness for Non-Forensic Personnel Investigators also avoid using anything at the scene, including bathrooms, sinks, phones, and towels, because doing so can both destroy evidence and deposit their own biological material.
Documenting Before Anything Moves
Investigators create a permanent visual record of the scene as they first encountered it before moving or collecting a thing. That means photographs, video, written notes, and measured sketches capturing the position and condition of every item of interest.2United Nations Office on Drugs and Crime (UNODC). Crime Scene and Physical Evidence Awareness for Non-Forensic Personnel Details like whether doors were open or closed, whether lights were on, and any odors present all get documented.
This step matters at trial. Defense attorneys routinely challenge whether evidence was in the location investigators claim, and a thorough visual record is often the strongest rebuttal. Once evidence is collected, the original scene can never be perfectly recreated.
Finding Hidden Biological Material
Not all biological evidence is visible. A bloodstain on a white carpet is obvious; cleaned-up blood, trace semen, or thin saliva residue can be invisible to the naked eye. Two tools handle most of that gap.
An alternate light source (ALS) emits specific wavelengths that cause certain biological fluids to fluoresce. In a darkened room, an ALS passed over surfaces makes semen, saliva, and some other body fluids glow, revealing deposits that would otherwise go unnoticed.3National Institute of Justice. Collecting DNA Evidence at Property Crime Scenes – Locating Evidence
For blood, investigators sometimes use luminol, a chemical reagent that reacts with the iron in hemoglobin and produces a blue-green glow. Luminol is particularly useful for blood that has been wiped or washed away, because trace amounts remaining after cleaning still trigger the reaction.3National Institute of Justice. Collecting DNA Evidence at Property Crime Scenes – Locating Evidence Anything detected through ALS or luminol gets flagged, collected, and labeled as a biological sample for laboratory testing.
Collection Methods
Collection technique varies with the type of evidence, the surface it sits on, and its condition. Every tool used is sterile, and investigators change gloves between samples to prevent cross-contamination.
Swabbing
Swabbing is the workhorse method for blood, saliva, and other fluids on hard surfaces. The standard approach is the wet-dry technique. An investigator moistens a sterile cotton swab with a drop or two of distilled water, then firmly rubs the stained area while rotating the swab to collect material evenly. A second, dry swab follows immediately, using the same pressure and rotation on the same area. The wet swab loosens cellular material; the dry swab picks up whatever the first pass dislodged.4National Institute of Justice. Collecting DNA Evidence at Property Crime Scenes – Second Priority
Touch DNA from surfaces like door handles, cell phones, or weapon grips is collected similarly, though investigators sometimes use adhesive tape or specialized lifting techniques when swabbing isn’t effective for a particular surface.
Cutting and Scraping
When a biological stain sits on fabric, carpet, or another material that can be sectioned, investigators cut out the stained area using a sterile blade. For dried material on hard surfaces, scraping with a clean razor blade or scalpel onto a piece of paper can outperform swabbing. The sample is then air-dried before packaging, and investigators never use heat, fans, or direct sunlight to speed up drying.5National Institute of Justice. Collecting DNA Evidence at Property Crime Scenes – Procedures
Whole-Item Collection
Sometimes the smartest move is to bag the entire item. A cigarette butt, a piece of chewing gum, a used condom, or a discarded drink container is easier to handle intact than to swab in the field. Small items get packaged individually and sent to the lab, where analysts can work under controlled conditions with better equipment.
Wet-Vacuum Systems
Porous surfaces like rough concrete, unfinished wood, or brick don’t give up much to a swab. Wet-vacuum collection systems spray sterile solution onto the surface under pressure while simultaneously vacuuming it back up along with any DNA present. The collected solution then passes through a filter to concentrate the biological material. Studies comparing this approach to traditional swabbing on porous surfaces found the wet-vacuum recovered substantially more DNA, sometimes more than ten times the amount. Rule of thumb: if the stain is visible, swabbing works; if it’s invisible or the surface is rough and absorbent, wet-vacuum may be the better option.
Sexual Assault Evidence Kits
Sexual assault cases follow a specialized DNA collection protocol performed during a forensic medical exam. A trained examiner takes swabs from specific body areas based on the victim’s account: oral swabs of the gum line, spaces between teeth, and under the tongue when oral contact is reported, and genital and anal swabs targeting the areas described. The examiner also takes a reference swab from the inside of the victim’s cheek so the lab can distinguish the victim’s DNA from the perpetrator’s.
The kit may also include the victim’s clothing, especially undergarments, fingernail swabs if the victim reports scratching the attacker, and swabs from any body area where the attacker may have made contact through saliva or other fluids. When drug-facilitated assault is suspected, blood and urine samples are collected to test for substances. Many of these samples yield the best results when collected within 24 to 48 hours of the assault.
Reference and Elimination Samples
Not every DNA sample at a crime scene belongs to the perpetrator. A homeowner’s blood from a cooking accident, a roommate’s saliva on a shared coffee mug, or a first responder’s skin cells from an accidental touch can all show up in lab results. To sort through it, investigators collect elimination samples from people who had legitimate access to the scene. If those samples match DNA found on an item, the lab can rule that person out and focus on unknown profiles.6National Institute of Justice. Principles of Forensic DNA for Officers of the Court – Elimination Samples
The collection method for a reference or elimination sample is a buccal swab, a quick and painless rub of a cotton or foam-tipped applicator along the inside of the cheek. The swab picks up loose cells from the cheek lining, producing a clean, high-quality DNA sample. It is then air-dried, sealed in a paper envelope with evidence tape, and labeled with the individual’s name, the date, and the collector’s initials. Elimination samples submitted voluntarily are not entered into the national DNA database.7Office of the Law Revision Counsel. United States Code Title 34 Section 12592 – Index to Facilitate Law Enforcement Exchange of DNA Identification Information
How DNA Evidence Is Preserved
DNA is durable but not indestructible. Heat, humidity, and microbial activity break DNA strands into progressively smaller fragments, eventually making a sample useless for profiling. Moisture is the biggest enemy: wet biological material that stays damp becomes a breeding ground for bacteria that consume the DNA itself.8Office for Victims of Crime (OVC). Understanding DNA Evidence: A Guide for Victim Service Providers Outdoors, the clock moves fast. Research in temperate climates found that blood and saliva samples exposed to the elements for more than three months rarely produced complete profiles, and none did after twelve months. Skin cell deposits sometimes become unreadable within two weeks of full outdoor exposure, and direct contact with soil accelerates the process. Evidence collected in the first hours after a crime is almost always higher quality than evidence collected days later.
Paper, Not Plastic
How evidence is packaged after collection determines whether it survives intact until trial. The cardinal rule: biological evidence goes in paper, never plastic. Paper bags and envelopes let moisture evaporate, keeping samples dry. Plastic traps humidity and creates the warm, moist environment bacteria need to break down DNA.8Office for Victims of Crime (OVC). Understanding DNA Evidence: A Guide for Victim Service Providers
Each item is packaged separately to prevent cross-contamination between samples. Every package gets a label with the case number, item number, collection date and location, a description of the evidence, and the collector’s initials. The package is sealed with evidence tape, and the collector signs and dates across the seal so any tampering is immediately visible.
Temperature and Humidity
For short-term storage, refrigeration between 2°C and 8°C (roughly 36°F to 46°F) with humidity below 25% is the recommended standard for both liquid blood and dried swabs.9National Institute of Standards and Technology. Biological Evidence – Storage Conditions Liquid blood can survive at a controlled room temperature for less than 24 hours in transit, but refrigeration is preferred. Wet evidence that cannot be dried before transport should be refrigerated or frozen as soon as possible. Properly stored dried samples kept at room temperature in paper packaging remain viable for extended periods, and refrigerated or frozen samples hold up even longer.8Office for Victims of Crime (OVC). Understanding DNA Evidence: A Guide for Victim Service Providers
Chain of Custody
From the moment a sample is collected until it’s presented in court, every person who touches it is documented. The chain of custody record includes who handled the evidence, when they received it, when they passed it along, and what they did with it. A break in the chain, meaning any period where the evidence’s location or handler is unaccounted for, gives defense attorneys grounds to argue the evidence was tampered with or contaminated.
The chain begins at the crime scene with the collector’s signature on the evidence packaging and continues through transport, laboratory receipt, analysis, storage, and eventual presentation at trial. In practice, investigators keep meticulous transfer logs, labs sign for incoming evidence, and every movement of a sample between analysts is recorded. Cases have been thrown out over missing signatures.
When Investigators Can Legally Collect Someone’s DNA
Collecting someone’s DNA is a search under the Fourth Amendment. In most circumstances, law enforcement needs either a warrant supported by probable cause or the person’s voluntary consent to collect a sample. Consent must be genuinely voluntary; a person who agrees to give a sample cannot have been coerced or misled about what it will be used for.
The major exception involves arrestees. In 2013, the Supreme Court held in Maryland v. King that taking a cheek swab from someone arrested for a serious offense is a reasonable search under the Fourth Amendment, comparable to fingerprinting and photographing during booking.10Justia Law. Maryland v. King, 569 US 435 Every state has a DNA collection law for convicted offenders, and a majority also authorize collection from arrestees, though the qualifying offenses vary.
DNA left in a public place is treated differently. Under federal constitutional law, there is no Fourth Amendment protection for biological material someone abandons, such as a coffee cup thrown in a public trash can or a cigarette butt dropped on a sidewalk. Investigators can collect and test that material without a warrant.11National Institute of Justice. Principles of Forensic DNA for Officers of the Court – Resources A handful of states impose tighter restrictions under their own constitutions, and some apply a “forced abandonment” doctrine that bars police from using items discarded during an unlawful pursuit. As a general rule, though, once you throw something away in public, you’ve given up your expectation of privacy in whatever DNA it carries.