Viral Load, Detection Rate and Sensitivity of Nasopharyngeal and Oropharyngeal Swab Sampling for Diagnosis of COVID-19

The rapid spread of the COVID-19 pandemic has led to a major public health crisis. Accurate screening methods for COVID-19 infection is essential and crucial for case detection, isolation, prevention and control of the current pandemic. At present, nasopharyngeal and oropharyngeal swabs are typically used as the method of choice for the diagnosis of SARS-CoV-2 infection. We carried out a review on the accuracy of the two different sampling sites, the nasopharyngeal and oropharyngeal swab sampling, focusing on the viral load, detection of positive cases and sensitivity in real-time polymerase chain reaction (RT-PCR) assay in diagnosing COVID-19. A total of 25 articles related to the topic were selected out of 5221 articles searched online using Scopus, PubMed and Medline, Embase, Web of Science, and Google scholar with the keywords COVID-19, SARS-CoV-2, nasopharyngeal swab, oropharyngeal swab, nasal swab and throat swab. All full text original articles were obtained and reviewed. Nasopharyngeal swab had significantly higher SARS-CoV-2 load than oropharyngeal swab (mean Ct value ranging from 24.3-37.8, higher detection of positive rate (highest rate 62.5%) and sensitivity (highest sensitivity 98.3%, P<0.05) in RT-PCR assay compared to oropharyngeal swab. Based on the scientific literature review, both nasopharyngeal and oropharyngeal swabs were reported to have 30% probability of yielding false negative results; thus clinically suspicious patients with negative results should be viewed with concern. In conclusion, although several methods of COVID-19 screening and type of specimen are available, nasopharyngeal swab is the best option for large scale screening as it yields significantly higher viral load, higher detection of positive rate among cases and higher sensitivity in RT-PCR assay compared to oropharyngeal swab in detecting SARS-CoV-2.