Background:In routine clinical practice diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS -CoV-2) in- fections is done by reverse-transcription PCR (RT-PCR). Ct (cycle threshold) value represents the cycle of PCR when fluo- rescence crosses the threshold of detection and has an inverse relationship with the initial load of cDNA in the case of coronavirus. This study is being done to evaluate the diagnostic relevance of Rapid antigen test (Accucare COVID -19 antigen card test) for COVID-19 as compared to SARS-CoV-2 RT-PCR (Q-Line Molecular ER nCOV-19 RT-PCR Kit) with spe- cial reference to RdRp Ct value.
Methods:Overall 5314 nasopharyngeal swabs in pair were collected from patients within the duration of September and October 2020, who came to IGIMS Patna for consultation. We performed rapid antigen test and RT -PCR test after RNA extraction from an automated system (MagNA Pure 96) as per manufacturer protocol by using the above- mentioned kits.
Results:Among all the samples tested, 102 (1.91%) samples were found positive in the rapid antigen test and 229 (4.30%) in the RT PCR test. Among positive RT-PCR result, Ct value of 91 samples were ≤ 20, those of 44 samples were between 21 to 25 and those of 94 samples were ≥26. We also observed that about 98% of samples with Ct value ≤ 20 for the RdRp gene were positive for rapid antigen, whereas only 25 % were positive with rapid antigen test having Ct value between 21 to 24. No sample, having Ct value ≥25 for RdRp gene, was found to be positive by rapid antigen test.
Conclusions:RT-PCR is considered a gold standard test for diagnostic confirmation of COVID-19. Rapid antigen test is ideal for screening and isolation of patient having a high viral load which is more infectious and thus limit the spread of infection in communities.