2. Oropharyngeal swab
The oropharyngeal swab (OPS) is also known as the “throat swab”. It refers to the sampling of one or more of the four oropharyngeal subsites (tonsils, soft palate, base of tongue and posterior pharyngeal wall). Just as the nasopharynx is paired posterior to the nasal cavity, the oropharynx is situated behind the oral cavity and commences at the hard-soft palate transition. The commonest target site of an OPS is the posterior pharyngeal wall as it considered the anatomical continuum of the nasopharynx. Throat swabs or OPS are commonly used for respiratory sampling in the current COVID-19 outbreak. An early landmark study detailing the clinical characteristics of Wuhan COVID-19 patients used OPS as the sole respiratory sampling method.7 In the largest study to date of a Chinese non-Wuhan COVID-19 cohort in Zhejiang, OPS was similarly used as the sole sampling method.8 However, low negative predictive value of OPS has been reported. Xie et al reported that only 9 out of 19 (47%) OPS from ultimately seropositive COVID-19 patients were positive, calling to attention the importance of repeated sampling from multiple sites, including the lower respiratory system, to increase diagnostic yield.9
The NS, NPS, NPA and OPS methods were compared in a Chinese study by Ye et al, which reviewed SARS, MERS, and H1N1 respiratory sampling literature and concluded that among all the upper respiratory sampling methods, nasopharyngeal aspirate (NPA) had a higher positive rate within 2 weeks of symptom onset, while combined nasal and oropharyngeal swabs (NS + OPS) was the least harmful to medical staff during sampling.10 It remains to be evaluated if the above findings can be extrapolated to SARS-CoV-2.