Cotton swabs continue to be the primary choice for biological evidence collection, although swab composition technology has advanced and other swab choices are available. Cotton may not be the best choice for every situation, but practitioners do not always know which alternatives to use.
A recent article written by National Institute of Justice staff[1] describes the results of a systematic literature review and analysis to determine which swab types work best for collecting biological evidence. The authors reviewed findings from 39 research studies to determine the best swab types based on the DNA’s source and the surface (substrate) it is on.
The literature identified five DNA sources: blood, saliva, semen, touched surfaces, and acellular DNA and tested 17 different kinds of porous (e.g. brick, paper, skin, drywall, and textiles) and nonporous (e.g. glass, metal, plastic, stone, and ammunition) substrates.
The studies’ researchers analyzed 41 substrate-DNA source combinations and identified a best-performing swab type for 13 combinations. For 58% of the substrate-DNA source combinations, the NIJ report identified a best performing swab type, swab types that perform similarly, or a swab type that performs based on the volume of DNA source.
The study’s authors made three additional conclusions:
- The overall workflow needs to be considered according to swab type: The best swab type depends on the entire process, extraction through DNA typing. Some studies’ results depended on the extraction chemistry or protocol used to process the swab.
- Swabs made of the same material do not always perform similarly: All swabs made of the same material do not have similar performances. Multiple studies evaluating multiple swabs of the same material sometimes saw a range in performance.
- Swab users do not affect the results: Two studies had a sufficiently large population size to find no significant differences among people who performed swabbing operations.
The NIJ report provides guidance to researchers, practitioners, and crime scene personnel in terms of selecting swab materials and brands and observing how the overall workflow influences swab performance. It also highlights research gaps where future studies can analyze additional substrate-DNA source combinations.
Find the Best Option Based on DNA Source, Substrate, and Swab Type
Not researched - the substrate-DNA source combination has not been researched and/or disseminated in a journal article
Researched - the substrate-DNA source combination research has been disseminated in at least one journal article
Not significantly different - the substrate-DNA source combination has been researched, met the analysis criteria, yet statistics show no significant differences exist between swab types
Best performing - the substrate-DNA source combination has been researched, analyzed with the analysis criteria, and statistics show that the swab type performs best compared to the others researched
Volume dependent - the substrate-DNA source combination has been researched, met the analysis criteria, yet swab type results vary based on the volume of the DNA source being swabbed
Blood
| Substrate | Cotton | Nylon | Rayon | Polyester | Proprietary Dissolvable Polymer | Bicomponent Fibers | Foam | Microfiber |
|---|---|---|---|---|---|---|---|---|
| Directly on Swab | Best performing | Volume dependent | Researched | Researched | Volume dependent | Not researched | Researched | Not researched |
| Drywall | Not significantly different | Not researched | Not researched | Not researched | Not researched | Not significantly different | Not significantly different | Not significantly different |
| Wood - Unfinished | Researched | Researched | Researched | Researched | Not researched | Not researched | Researched | Not researched |
| Brick | Researched | Researched | Researched | Researched | Not researched | Not researched | Researched | Not researched |
| Textiles | Not significantly different | Researched | Researched | Not researched | Not researched | Not significantly different | Not significantly different | Not significantly different |
| Leather | Researched | Researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Paper | Researched | Researched | Not researched | Not researched | Not researched | Researched | Researched | Researched |
| Vagina | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Skin | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Glass | Not significantly different | Not significantly different | Not significantly different | Researched | Not researched | Not significantly different | Not significantly different | Not significantly different |
| Plastic - Smooth | Researched | Researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Plastic - Textured | Researched | Researched | Researched | Researched | Not researched | Not researched | Researched | Not researched |
| Stone, Outdoor Flooring (Tile) | Researched | Researched | Researched | Researched | Not researched | Not researched | Not researched | Not researched |
| Wood - Finished | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Metal (Handles, Blades, Cans) | Researched | Researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Explosive Devices and Components | Best performing | Researched | Not researched | Not researched | Not researched | Not researched | Researched | Not researched |
| Ammunition and Magazines | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
See the article Evaluation of swab materials in forensic DNA testing: a systematic review by NIJ staff in the Egyptian Journal of Forensic Sciences.
Saliva
| Substrate | Cotton | Nylon | Rayon | Polyester | Proprietary Dissolvable Polymer | Bicomponent Fibers | Foam | Microfiber |
|---|---|---|---|---|---|---|---|---|
| Directly on Swab | Researched | Volume dependent | Researched | Researched | Volume dependent | Researched | Researched | Not researched |
| Drywall | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Wood - Unfinished | Researched | Researched | Researched | Researched | Not researched | Not researched | Best performing | Not researched |
| Brick | Researched | Volume dependent | Volume dependent | Researched | Not researched | Not researched | Volume dependent | Not researched |
| Textiles | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Leather | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Paper | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Vagina | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Skin | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Glass | Best performing | Best performing | Researched | Researched | Not researched | Not researched | Researched | Not researched |
| Plastic - Smooth | Volume dependent | Volume dependent | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Plastic - Textured | Not significantly different | Not significantly different | Researched | Researched | Not researched | Not researched | Not significantly different | Not researched |
| Stone, Outdoor Flooring (Tile) | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Wood - Finished | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Metal (Handles, Blades, Cans) | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Explosive Devices and Components | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Ammunition and Magazines | Researched | Best performing | Not researched | Not researched | Not researched | Not researched | Researched | Not researched |
See the article Evaluation of swab materials in forensic DNA testing: a systematic review by NIJ staff in the Egyptian Journal of Forensic Sciences.
Semen
| Substrate | Cotton | Nylon | Rayon | Polyester | Proprietary Dissolvable Polymer | Bicomponent Fibers | Foam | Microfiber |
|---|---|---|---|---|---|---|---|---|
| Directly on Swab | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Drywall | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Wood - Unfinished | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Brick | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Textiles | Researched | Researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Leather | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Paper | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Vagina | Researched | Researched | Best performing | Not researched | Not researched | Not researched | Not researched | Not researched |
| Skin | Researched | Best performing | Not researched | Not researched | Not researched | Best performing | Not researched | Not researched |
| Glass | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Plastic - Smooth | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Plastic - Textured | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Stone, Outdoor Flooring (Tile) | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Wood - Finished | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Metal (Handles, Blades, Cans) | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Explosive Devices and Components | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Ammunition and Magazines | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
See the article Evaluation of swab materials in forensic DNA testing: a systematic review by NIJ staff in the Egyptian Journal of Forensic Sciences.
Touched Surface
| Substrate | Cotton | Nylon | Rayon | Polyester | Proprietary Dissolvable Polymer | Bicomponent Fibers | Foam | Microfiber |
|---|---|---|---|---|---|---|---|---|
| Directly on Swab | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Drywall | Researched | Not researched | Not researched | Not researched | Not researched | Best performing | Researched | Researched |
| Wood - Unfinished | Researched | Researched | Researched | Researched | Not researched | Not researched | Researched | Not researched |
| Brick | Researched | Researched | Researched | Researched | Not researched | Not researched | Researched | Not researched |
| Textiles | Not significantly different | Best performing | Not researched | Not researched | Not researched | Not significantly different | Not significantly different | Not significantly different |
| Leather | Researched | Best performing | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Paper | Not significantly different | Researched | Not researched | Not researched | Not researched | Not significantly different | Not significantly different | Not significantly different |
| Vagina | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Skin | Not significantly different | Not significantly different | Researched | Not researched | Not researched | Not researched | Researched | Not researched |
| Glass | Not significantly different | Not significantly different | Researched | Researched | Not researched | Not significantly different | Not significantly different | Not significantly different |
| Plastic - Smooth | Not significantly different | Not significantly different | Not researched | Researched | Not researched | Not researched | Researched | Not researched |
| Plastic - Textured | Not significantly different | Not significantly different | Researched | Best performing | Not researched | Not researched | Not significantly different | Not researched |
| Stone, Outdoor Flooring (Tile) | Researched | Researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Wood - Finished | Researched | Researched | Not researched | Researched | Not researched | Not researched | Researched | Not researched |
| Metal (Handles, Blades, Cans) | Researched | Researched | Researched | Researched | Not researched | Not researched | Researched | Not researched |
| Explosive Devices and Components | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Ammunition and Magazines | Best performing | Best performing | Not researched | Not researched | Not researched | Not researched | Researched | Not researched |
See the article Evaluation of swab materials in forensic DNA testing: a systematic review by NIJ staff in the Egyptian Journal of Forensic Sciences.
Acellular DNA
| Substrate | Cotton | Nylon | Rayon | Polyester | Proprietary Dissolvable Polymer | Bicomponent Fibers | Foam | Microfiber |
|---|---|---|---|---|---|---|---|---|
| Directly on Swab | Researched | Not significantly different | Best performing | Not researched | Not significantly different | Not researched | Not researched | Not researched |
| Drywall | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Wood - Unfinished | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Brick | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Textiles | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Leather | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Paper | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Vagina | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Skin | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Glass | Researched | Researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Plastic - Smooth | Researched | Researched | Researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Plastic - Textured | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Stone, Outdoor Flooring (Tile) | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Wood - Finished | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Metal (Handles, Blades, Cans) | Researched | Researched | Researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Explosive Devices and Components | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
| Ammunition and Magazines | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched | Not researched |
See the article Evaluation of swab materials in forensic DNA testing: a systematic review by NIJ staff in the Egyptian Journal of Forensic Sciences.