To continue our recent discussions on disease testing, today we’re talking about swabbing and swabs.
Perhaps you’ve been lucky enough to experience a swab test firsthand. As testing becomes more widespread, we are all likely to have a nasopharyngeal swab taken for a COVID-19 test. When I was a child, maybe around seven, I remember going to the doctor’s office to be tested for influenza, another virus that replicates in the nasopharynx. When the nurse first pulled the slim, wire swab out of the packaging, I thought no way. No way that daunting, 5-cm long wire will fit up my nose. Sadly, I was mistaken. I won’t lie – it wasn’t the most pleasant experience. The tickling sensation lingered in my throat long after the swab had been removed. I still felt it even after my results had come back – negative, luckily for me. Just a common cold!
This kind of swab is referred to as a nasopharyngeal swab, which I learned many years later. Nasopharyngeal swabs are used to collect samples from the back wall of the nasopharynx (hence the name), which is where the nasal passages meet the throat. Other common swab tests include nasal swabs and oropharyngeal (throat) swabs.
Swabs are made out of a number of different materials. Since the nasopharynx is much farther back in the nasal passage, the swab needed to get there must be longer and somewhat bendable. The swabs used for sampling the nostrils and throat are generally stiffer and shaped like long Q-tips. The materials that comprise the tips of these swabs are different, too. If you are trying to get material out of the oral or nasal cavities for testing, you need a surface at the tip that is good at grabbing that material. It is also important to be able to wash the material off and into the testing solution when needed. Thus, regular cotton is often not suitable.
In terms of COVID-19 testing, the swabs you’re most likely to encounter are nasopharyngeal and nasal swabs. Nasal swabs are less invasive. The swab only needs to be inserted 1 cm into the nostril and rubbed along the septum for a few seconds. Nasopharyngeal swabs, on the other hand, require a longer swab, which is inserted about 4-6 cm back into the nostrils (about half-way between the entrance to the nares and the base of the ear). The swab is then rotated inside the nasal passages and left in place for a few seconds to absorb the sample.
Swabs made to sample the nasopharynx usually have a tip made of plastic foam, or another material with lots of surface area. These are called “flocked” swabs. Only a few factories in the world make these swabs, which is why you regularly hear about swab shortages for testing. Sadly, using a version of a regular Q-tip would not be a suitable replacement. There are a number of innovative groups around the world looking for ways around this shortage. Some of them include 3D printing swabs made from medical grade plastics. All swabs that are used to apply topical medications or collect fluid samples are considered Class I medical devices by the FDA.
In studies comparing the sensitivity of nasal vs nasopharyngeal swabs for influenza, nasopharyngeal swabs were found to be slightly more sensitive (94% vs. 89% sensitivity). This means that for the flu, a nasal swab sample will lead to a false-negative test result more often than a nasopharyngeal swab. But despite their inferior sensitivity, nasal swabs are simpler and useful for “self-swabbing”- taking your own sample to send or have delivered to a testing facility. According to the most recent update to the CDC’s COVID-19 specimen collection guidelines, nasal swabs taken by a healthcare worker or via self-swabbing are acceptable if taking a nasopharyngeal swab is not possible. But these guidelines are sure to change as more information comes out about how swab type affects false-negative rates for COVID-19. Early indications are that nostril swabs are not as good as nasopharyngeal swabs for this new virus.