Effect of storage temperatures simulating transport conditions of nasopharyngeal swabs on the results of a chemiluminescence immunoassay (CLIA) to detect severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antigen

Abstract

Purpose: Chemiluminescence Immunoassay (CLIA) is high throughput, rapid diagnostic test which has recently come up for the detection of SARS-CoV-2 antigen. The present study evaluated performance of CLIA antigen test in nasopharyngeal swab samples stored at different temperatures for 7 days to simulate the transport conditions and transit time across the country from remote peripheral laboratories to central facilities.

Materials and methods: Limit of detection (LOD), sensitivity and specificity of VITROS® SARS-CoV-2 antigen assay was determined using Real-time reverse transcriptase PCR (rRT-PCR) confirmed SARS-CoV-2 positive and negative samples. To detect the effect of storage temperatures on VITROS ®SARS-CoV-2 antigen results, samples were stored at 4 ​°C, 25 ​°C & 37 ​°C for 7 days followed by detection of SARS-CoV-2 nucleocapsid antigen and compared with N-gene rRT-PCR.

Results: The VITROS® SARS-CoV-2 antigen test was found to have a sensitivity and specificity of 78.9% and 100% respectively with high sensitivity of 88.1% for samples with Ct ​< ​30. The LOD of VITROS assay was equivalent to 3800 copies of RNA per reactions as compared to 72 copies per reaction for rRT-PCR. We observed that more than 80% of samples with <30 Ct values could be detected by VITROS SARS-CoV-2 antigen assay at day 7 even when stored at 37 ​°C. For samples with Ct values between 26 and 30, on day 7 the positivity rate of N-antigen at 4 ​°C was 90.9% and 37 ​°C was 63.6%.

Conclusions: CLIA testing can be carried out for the detection of SARS-CoV-2 N-protein in NP-swab samples transported in cold chain even with 7 days transit time, particularly for Ct ​< ​30 samples which represents cases with higher transmissibility. As drop in positivity for VITROS assay was lower as compared to rRT-PCR on day 7 in cold chain-maintained samples, the assay can be useful to screen samples received from remote peripheral areas before performing rRT-PCR.

Keywords: CLIA; SARS-CoV-2 antigen; Storage temperature; Transport.