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.