Swabs are used for collecting specimens for the recovery of organisms or point-of-care testing (POCTs). Swabs generally are poor specimens if tissue or needle aspirates can be obtained. In recent years, great strides have been made to improve organism recovery from collection swabs by changing swab composition and design.
Swabs today come in various materials, such as Dacron, nylon, polyurethane, rayon (polyester), and calcium alginate. The tips may be mesh, sponge, or flocked. The shafts are composed of wood, aluminum, or plastic.
History of Swab Use
The pathologist William Thomas Councilman developed the first cotton swab in 1893, and the method was published in the American Journal of Medical Science. In the 100 years since Councilman’s original publication, few innovations have been made, such as changes in fiber type and design to collect, retain and elute the maximum amount of sample materials.
Cotton fibers containing inhibitory fatty acids were replaced with calcium alginate fibers. As they also contain inhibitory substances for PCR, they were later replaced with non-toxic synthetic fibers, such as rayon and polyester (Dacron). In 1992, Dickinson patented the non-toxic polyurethane foam-tipped swabs, and Copan patented flocked swabs in 2004.
Types of Swabs
Commercially available swabs can differ in tip materials, such as cotton, calcium alginate, dacron, nylon, polyester, rayon, and polyurethane.
Different tip materials have different chemicals and physical characteristics, which might influence the specimen collection and release. Swabs differ in absorption capacity and efficiency of releasing nucleic acids and proteins.
The type of swabs used for sample collection is very important. Calcium alginate or dacron swabs are acceptable for obtaining nasopharyngeal swab specimens; calcium alginate is optimal for culture. Dacron or rayon swabs on plastic shafts are preferred for virology and nucleic acid-based testing.
Fatty acids in cotton swabs are toxic to Neisseria gonorrhoeae and Bordetella pertussis therefore, cotton swabs should never be used for culturing Bordetella or gonococci.
Cotton swabs are used to collect various types of specimens, including specimens for polymerase chain reaction (PCR) tests. Still, they are not recommended as residues present in cotton-tipped swabs inhibit PCR assays. A cotton swab moistened with phosphate-buffered saline is used to collect specimens from the base of the ulcer.
Calcium alginate swabs
Swabs made of calcium alginate are commonly used to collect nasopharyngeal specimens (excluding those specimens for chlamydia or viral culture). Calcium alginate fibers are toxic for tissue culture, inactivate certain viruses, such as HSV, and interfere with PCR and fluorescent-antibody tests.
Calcium alginate swabs are generally more toxic for gonococci and mycoplasmas than treated cotton swabs. Cotton and calcium alginate swabs are not recommended for microbiological diagnostics.
Dacron and rayon swabs contain non-toxic, hydrophilic synthetic fibers. Dacron swabs are the least toxic and most efficient at releasing antigens. Dacron swabs are also preferred swabs for molecular assays and viral specimens.
Dacron-tipped swabs on plastic shafts are acceptable for collecting most upper respiratory tract microorganisms, chlamydiae, and even genital Mycoplasmas.
Sterile rayon swabs with plastic shafts are suitable for collecting most upper respiratory tract microorganisms. Rayon-tipped swabs on a thin wire may also be used to collect specimens for isolation of mycoplasmas and chlamydiae.
Swabs tipped with nylon, polyester swabs, and polyurethane foams are also available.
The lack of sufficient specimen volume is the major limitation of swab usage. To overcome this limitation, newer swab designs are used. Swab tip structure may be flocked fiber, tightly wound, or knitted.
Newer swab designs, such as flocked swabs or sponges, are made to capture more specimens and, later, better release the sample into a transport medium or onto an agar plate.
Traditional fiber swab specimens have an internal mattress core that can trap organisms and may be vortexed (mixed) in 0.5 to 1 mL of saline or broth for 10 to 20 seconds to dislodge material from fibers.
A flocked swab has been designed to uptake a large volume of liquid sample. Flocked swabs contain no mattress core, sample stays close to the surface and elutes out rapidly and spontaneously.
Flocked swabs have short nylon fiber strands attached perpendicularly to the plastic applicator. Capillary hydraulics between the strands draws samples from the sampling site into this hydrophilic layer. Elution occurs when the swab is placed in the liquid medium supplied with the swab.
This improved design enhances the recovery of both aerobes and anaerobes. Flocked swabs are commercially available from numerous manufacturers.
Copan, Murrieta, CA, has introduced a liquid-based swab transport device named ESwab. Find more about ESwab.
Fiber Swab vs. Flocked Nylon Swab
The synthetic tip of a flocked swab is brush-like and has more surface area than a cotton swab. This offers better specimen collection (collection of more material). Flocked swabs offer rapid and complete elution (release) of the sample into liquid media.
|Features||Traditional Fiber Swab||Flocked Nylon Swab|
|Swab design||Yards of fiber are wrapped around an applicator||Nylon fibers are applied to the applicator using a proprietary flocking process.|
|Trapping of sample||The sample is trapped in swab fibers||The sample stays on the surface of the swab and completely elutes on contact with transport media.|
|Tests from one sample||One swab for one test||Up to ten aliquots for multiple tests|
|Sample available for testing||Less than 10%||Up to 90%|
Shafts Used in Swabs
Swab shafts are made up of aluminum or plastic, or wooden shafts. Swabs with plastic shafts are recommended for collecting bacteria, viruses, and mycoplasmas from mucosal membranes. The organisms are more easily removed from the plastic shafts than from other materials such as wooden shafts or wire.
Swabs with cotton tips and wooden shafts are not recommended for virology and molecular tests. Wooden shafts are toxic to many bacteria, such as Chlamydia trachomatis and viruses. Calcium alginate swabs with aluminum shafts are also used to amplify nucleic acids.
Performance of Different Swabs
Aleksandra Anna Zasada and the team tested the absorption capacity and efficiency of releasing nucleic acids and proteins of four common swabs (dacron, rayon, polyurethane foam, and flocked nylon swabs).
The highest absorption was seen with flocked nylon swabs.
Flocked swabs were also most efficient for DNA extraction, with extraction capacity >3.5 times higher than rayon swabs.
The most efficient protein recovery was measured from rayon swabs and dacron swabs.
References and further reading
- Zasada, A.A., Zacharczuk, K., Woźnica, K. et al. The influence of a swab type on the results of point-of-care tests. AMB Expr 10, 46 (2020). https://doi.org/10.1186/s13568-020-00978-9
- Centers for Disease Control and Prevention (2017) Best practices for healthcare professionals on the use of polymerase chain reaction (PCR) for diagnosing pertussis. https://www.cdc.gov/pertussis/clinical/diagnostic-testing/diagnosis-pcr-bestpractices.html. Accessed 6 November 2022.
- Goldfarb DM, Slinger R, Tam RK, Barrowman N, Chan F. Assessment of flocked swabs for use in identification of streptococcal pharyngitis. J Clin Microbiol. 2009 Sep;47(9):3029-30. doi: 10.1128/JCM.01163-09. Epub 2009 Jul 15. PMID: 19605581; PMCID: PMC2738109.
- Rapid Microbiology (2010) Flocked swabs proven superior in sample uptake and release. https://www.rapidmicrobiology.com/news/flocked-swabs-proven-superior-in-sample-uptake-and-release. Accessed 6 November 2022.