Antigen-detecting rapid diagnostic tests (Ag-RDTs) play a significant role in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) testing. Most Ag-RDTs currently used require nasopharyngeal swab samples. However, collecting nasopharyngeal swab samples requires qualified healthcare professionals, limiting the possibility to scaling up testing. The collection of nasopharyngeal swabs also causes discomfort for patients.
A diagnostic accuracy study comparing professional-collected anterior nasal and nasopharyngeal swabs
A team of researchers from various institutions in Germany and Switzerland recently performed a prospective diagnostic accuracy study, in which they compared professional-collected anterior nasal swabs to nasopharyngeal swabs, using WHO-listed SARS-CoV-2 Ag-RDT (STANDARD Q COVID-19 Ag Test, SD Biosensor) test kits. The findings of their study have been published on the preprint server, medRxiv.*
In this manufacturer-independent study, individuals who attended the ambulatory SARS-CoV-2 testing facility of Charité University Hospital Berlin, Germany, and were deemed to be at high risk of contracting COVID-19 infection, were enrolled between 11 and 18 November, 2020.
RT-PCR tests of a combined oro-/nasopharyngeal swab sample were used as the reference standard. The sensitivity and specificity of the tests to the different swabs and the percent of positive and negative agreement were assessed during the study. There were a total of 179 participants with an average age of 36.2 years. 48% of the participants were female and 14.5% of the participants had comorbidities.
“The study was continued until 30 positive NP swab samples according to Ag-RDT were obtained, which is the minimum recommended by the WHO Emergency Use Listing Procedure to demonstrate sample type equivalency.”
Results show over 90% agreement in positive and negative tests with both sampling techniques
The results showed that 41 (22.9%) tested positive for SARS-CoV-2 by real-time polymerase chain reaction reverse transcription reaction (RT-PCR). The positive percent agreement of the 2 different sampling techniques in the Ag-RDT was 93.5%. The negative percent agreement between the 2 techniques was 95.9%. The Ag-RDT with anterior nasal sampling had a sensitivity of 80.5% with 33 out of the 41 RT-PCR positives detected and had a specificity of 98.6% compared to that of RT-PCR.
No invalid results were observed in Ag-RDT in either anterior nasal or nasopharyngeal samples. Four patients tested positive by anterior nasal sampling, but not by nasopharyngeal sampling. One patient tested positive by nasopharyngeal sampling only.
The sensitivity with nasopharyngeal sampling was 73.2%, with 30 out of the 41 PCR positives detected, and the specificity was 99.3% compared to RT-PCR. In patients with a high viral load that is greater >7.0 log10 RNA SARS-CoV-2/swab, the sensitivity of the Ag-RDT with anterior nasal sampling was 100% and that for nasopharyngeal sampling was 94.7%.
“The strengths of the study are the standardized sampling methods, two independent blinded readers and an additional semi-quantitative assessment of Ag-RDT results.”
Anterior nasal samples show promise in reducing discomfort to patients and scaling up testing
This study attempts to demonstrate that the sensitivity of a WHO-listed SARS-CoV-2 Ag-RDT using a professional anterior nasal sampling kit is at least the same as that of the nasopharyngeal sampling kit. Interestingly, the differences in the IFUs of the 2 different test procedures could have resulted in different sensitivities.
Based on the results, the authors concluded that anterior nasal samples could be collected with less training, and the patient discomfort with this sampling method is relatively less. It therefore shows potential as a form of antigen testing strategy that can be rapidly and effectively scaled up. More studies of patient self-sampling are needed, however, to facilitate the scaling up of Ag-RDT testing.
“In conclusion, this study demonstrates that sensitivity of a WHO-listed SARS-CoV-2 Ag-RDT using professional AN-sampling kit is at least equal to that of NP-sampling kit, although confidence intervals overlap.”
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.