1) The proportion of positive results for nasopharyngeal and oropharyngeal samples were respectively:
- 19.0% vs 7.6% for all patients
- 7.3% vs 6.3% for patients under ambulatory care
- 32.9% vs 9.3% for hospitalized patients (in this case there was a high disparity between the results of the two tests)
2) The cumulative proportion of positive tests was calculated in the event of positivity for the nasopharyngeal and oropharyngeal samples (the results were slightly higher than those performed only on a nasopharyngeal sample):
- 21.5% of all patients,
- 9.9% of patients under ambulatory care,
- 35.4% of hospitalized patients.
3) Out of 27 positive tests per oropharyngeal sample, 18 nasopharyngeal samples were positive (66.7%) but out of 67 positive nasopharyngeal samples, 49 oropharyngeal samples were negative (73.1%)
Takeaways
73.1% of positive nasopharyngeal samples were negative when using the oropharyngeal sample.
Oropharyngeal swabs are correlated with a higher rate of false negatives and would, therefore, be recommended to use nasopharyngeal swabs to diagnose the presence of SARS-CoV-2 by RT-PCR.
This result is particularly important when patients' discharge from the hospital is based on two negative RT-PCR results on airway samples at least one day apart, as recommended by The WHO.
Compare the performance of nasopharyngeal and oropharyngeal swabs for the diagnosis of SARS-CoV-2 by RT-PCR
Method
Retrospective observational study conducted on 353 patients divided between hospitalized patients (n = 161) and patients with ambulatory care (n = 192) diagnosed with SARS-CoV-2 positive at Tongji hospital (Wuhan, China), from February,16th 2020 to March, 2nd, 2020.
The study involved patients for whom nasopharyngeal and oropharyngeal swabs had been taken at the time of diagnosis. The presence of SARS-CoV-2 was tested by RT-PCR and the test was considered positive for Ct value (cycle threshold) <40.