Performance Evaluation of Rapid Antigen Test for COVID-19 with SARS-CoV-2 RT-PCR using Nasopharyngeal Swab with Special Reference to RNA Dependent RNA Polymerase (RdRp) Cycle Threshold Values

SARS-CoV-2 is highly contagious, which spreads even by patients having no clinical symptoms or also from people suffering with only mild symptoms. The gold standard test for its diagnosis is reverse-transcription PCR (RT-PCR) but at times of pandemic, Rapid antigen tests (RAT) are required, which has a very less turn-around time. Evaluation of the performance of COVID-19 Rapid antigen test in comparison to SARS-CoV-2 RT-PCR using nasopharyngeal swab, in relation to RNA dependent RNA polymerase (RdRp) Cycle threshold (Ct) values. This observational and cross-sectional study was done on patients coming with features of Influenza-like illness (ILI) or for any aerosol generating procedure or on high-risk patients seeking hospitalization. Both RT-PCR and RAT for COVID-19 were done on samples collected from each patient and results were compared. Altogether, 5314 samples were tested, out of which 104 (01.95 %) & 229 (04.31 %) samples were found positive by the RAT & RT PCR test, respectively. Sensitivity, specificity, PPV and NPV of RAT were found to be 44.54%, 99.96%, 98.08% and 97.56%, respectively. 98.9 % of samples with Ct value <= 20 were positive by RAT, whereas only 2.2% samples having Ct value >= 26 were found to be positive. Cases having lower Ct values were found to be more symptomatic and vice-versa. RAT are not efficient in detecting the virus in samples showing high Ct values (Ct >= 26) by RT-PCR test. Patients with samples showing low Ct values (Ct <= 20) had more severe symptoms and vice-versa.