1. Nasal and nasopharyngeal specimens
The nasal swab (NS) refers to flocked swab stick sampling of the
anterior nasal cavity, which is bounded anatomically by the nares
anteriorly and the hard-soft palate transition posteriorly. In contrast,
the nasopharyngeal swab (NPS) involves the introduction of a flocked
swab stick deep into the nasopharynx (beyond the hard-soft palate
transition) to achieve direct contact with the posterior nasopharyngeal
mucosal wall. It is not possible to obtain a pure nasopharyngeal
specimen with a flocked swab stick as its introduction mandates contact
with the nasal cavity on the way in. It is, however, perfectly possible
to obtain a pure nasal swab, by avoiding deep insertion of the flocked
swab stick. The average length of the anterior nasal cavity in a healthy
adult is 5 to 7cm.4 A review of the English literature
on PubMed confirmed that there is no published comparison study
examining the SARS-CoV-2 viral load yield between nasal and
nasopharyngeal flocked swab specimens. A study of such nature is also
not clinically significant and would not have been logically undertaken.
In the 2003 severe acute respiratory syndrome (SARS) outbreak, an
alternative method of nasopharyngeal sampling known as “nasopharyngeal
aspirate” (NPA) was described by Chan et al.5 This
method allows for pure nasopharyngeal sampling without anterior nasal
cavity contamination. Instead of the flocked swab stick, a suction
catheter is threaded into the nasopharynx and suction is then activated,
aspirating nasopharyngeal mucus into a trap. Chan et al compared NPA
specimens with “nasal and throat swab” specimens and found a
marginally better performance of NPA in detecting SARS viral RNA in
confirmed SARS patients (NPA 29.6% vs NS/OPS 28.3%, no p-value
provided)5. It should be noted, however, that no
details of the “nasal and throat swab” procedure were available. It is
unclear if the “nasal” component referred to an anterior nasal cavity
sampling (NS) or a combined NS/NPS sampling. Contrary to Chan et al’s
results, another study conducted in Hong Kong during the 2003 SARS
outbreak reported that “pooled throat and nasal swabs” provided a
higher diagnostic yield compared to NPA specimens. It was further
highlighted that the former has the additional benefit of less risk of
generating aerosols since no suction is involved.6
In the current COVID-19 pandemic, literature surrounding NPA as a form
of respiratory sampling for SARS-CoV-2 viral shedding is lacking. Most
studies appear to utilize swabbing as the main method of upper
respiratory sampling. Comparisons of the combined nasal/nasopharyngeal
swab (NS/NPS) versus oropharyngeal swab (OPS) have been published. In a
study by Zou et al, 72 NS/NPS specimens sampled from the middle
turbinate and nasopharynx, across various days of illness, were analyzed
in eighteen COVID-19 patients. Higher viral loads (cycle threshold
[Ct] values) were detected in these specimens, compared to 72 OPS
specimens in the same group of patients.7 This
technique is time-consuming and resource intensive, and in the authors’
view, unsuitable for mass testing in a pandemic situation.