A method comparison study of the high throughput automated HISCL® SARS-CoV-2 antigen assay using nasopharyngeal swab samples from symptomatic and asymptomatic subjects against conventional RT-PCR

Our study aim was to evaluate the performance of the automated Sysmex HISCL® severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antigen assay against reverse-transcription polymerase chain reaction (RT-PCR). We tested 277 remnant frozen nasopharyngeal swab samples, stored in universal transport medium (UTM), yielding a sensitivity of 94.9% against historical RT-PCR results with cycle threshold (C_t) < 30, and a sensitivity of 76.7% for C_t < 35, and specificity of 100% (all C_t values) confirming compatibility of UTM-diluted samples with the assay system. Thereafter, we prospectively collected 141 nasopharyngeal swab samples in UTM from healthcare workers and 1369 paired swabs (400 UTM; 969 dry) from individuals at a public health testing center, with the first swab (UTM) reserved for RT-PCR, yielding a positivity rate of 4.6%. HISCL assay performance using UTM swabs was superior to dry swabs, with a sensitivity of 100% (95% confidence interval [CI] 71.5%–100%) at C_t < 30 versus 92.3% (95%CI 81.5%–97.9%), and a specificity of 99.3% (95% CI 98.1–99.89) against 83.3% (95%CI 80.7%–85.6%). We conclude that this antigen assay is suitable for high throughput facilities where the primary indication for testing is to rule out infection with low RT-PCR C_t values (proxy for high viral loads) to curb viral spread.