Head-to-head comparison of anterior nares and nasopharyngeal swabs for SARS-CoV-2 antigen detection in a community drive-through test centre in the UK

Objective To conduct a head-to-head diagnostic accuracy evaluation of anterior nares (AN) and nasopharyngeal (NP) swabs for SARS-CoV-2 antigen detection using two brands of rapid diagnostic tests (Ag-RDT).Methods Two prospective diagnostic evaluations were carried out at different time points and participant cohorts to evaluate the performance of paired AN and NP swabs in two Ag-RDT brands: Sure-Status (PMC, India) and Biocredit (RapiGEN, South Korea). The sensitivity and specificity of AN and NP swabs for each of the index test cohorts were calculated against the reverse transcription quantitative PCR (RT-qPCR) TaqPath COVID-19 (ThermoFisher, UK) using NP swabs as reference standard.Results A total of 372 participants were recruited for the Sure-Status cohort and 232 for the Biocredit, of which 119 (32.1%) and 122 (53.7%) were SARS-CoV-2 positive by RT-qPCR, respectively. Sensitivity and specificity of AN swabs were equivalent to those obtained with NP swabs in both cohorts: 83.9% (95% CI 76.0-90.0) and 98.8% (95% CI 96.6-9.8) using NP swabs and 85.6% (95% CI 77.1-91.4) and 99.2% (95% CI 97.1-99.9) with AN swabs for Sure-Status and; 81.2% (95% CI 73.1-87.7) and 99.0% (95% CI 94.7-86.5) with NP swabs and 79.5% (95% CI 71.3-86.3) and 100% (95% CI 96.5-100) with AN swabs for Biocredit. The agreement of the AN and NP swabs was high for both brands with an inter-rater reliability (kappa) of 0.918 and 0.833 for Sure-Status and Biocredit, respectively. The overall 50% limits of detection (LoD50) and 95% LoD (LoD95) were 0.9-2.4x104 and 3.0-3.2x108 RNA copies/mL for NP swabs and 0.3-1.1x105 and 0.7-7.9x107 RNA copies/mL for AN swabs, with no significant difference in LoD for any of the swab types or test brands.Conclusions The diagnostic accuracy of the two SARS-CoV-2 Ag-RDT brands evaluated in this study was equivalent using AN swabs than NP swabs. However, test line intensity was lower when using AN swabs, which could negatively influence the interpretation of the Ag-RDT results by lay users.Trail registration number NCT04408170.