RESUMO
The diagnostic of SARS-CoV-2 infection relies on reverse transcriptase polymerase chain reactions (RT-PCR) performed on nasopharyngeal (NP) swabs. Nevertheless, false negative results can be obtained with inadequate sampling procedures making the use of other matrices of interest. This study aims at evaluating the kinetic of serum N antigen in severe and non-severe patients and compare the clinical performance of serum antigenic assays with NP RT-PCR. Ninety patients were included and monitored for several days. Disease severity was determined according to the WHO clinical progression scale. The serum N antigen was measured with a chemiluminescent assay (CLIA) and the Single Molecular Array (Simoa). Thresholds for severity were determined. In severe patients, the peak antigen response was observed 7 days after the onset of symptoms followed by a decline. No peak response was observed in non-severe patients. Severity threshold for the Simoa and the CLIA provided positive likelihood ratio of 30.0 and 10.9 for the timeframe between day 2 and day 14, respectively. Compared to NP RT-PCR, antigenic assays were able to discriminate the severity of the disease (p = 0.0174, 0.0310 and p = 0.1551 with the Simoa, the CLIA and the NP RT-PCR, respectively). Sensitive N antigen detection in serum thus provides a valuable new marker for COVID-19 diagnosis and evaluation of disease severity. When assessed during the first 2 weeks since the onset of symptoms, it may help in identifying patients at risk of developing severe COVID-19 to optimize better intensive care utilization.
