Implementation of rapid SARS-CoV-2 antigenic testing in a laboratory without access to molecular methods: Experiences of a general hospital

Abstract

Background: The COVID-19 Ag (Antigen) Respi-Strip assay is a new immunochromatographic diagnostic tool recently available for antigenic diagnosis of SARS-CoV-2. The proposed sensitivity is not higher than 60 %, but its high specificity allows both quick decisions for the management of patients and confirmation by molecular diagnosis for only negative tests. However, from the first tests performed, we suspected that the sensitivity observed with routine use was much lower than that announced by the manufacturer.

Materials and methods: Over a period of one month, we compared the negative results obtained with the COVID-19 Ag Respi-Strip kit with those obtained from qRT-PCR performed in a laboratory qualified for the molecular diagnosis of SARS-CoV-2. All samples tested were naso-pharyngeal smears from UTM-RT medium.

Results: Of 774 patients tested, 714 negative samples were sent for confirmation, and 159 were found to be positive by qRT-PCR. The median positive percentage agreement was 23.9 % (95 % CI: 14.2 %-38.2 %). The Cohen's kappa score was 0.35.

Conclusion: Using this immunochromatographic assay as a triage test did not significantly reduce the number of samples outsourced for COVID-19 confirmation by qRT-PCR. In addition, even if the turn-around time is short, the assay is completely manual, which is not suitable for large volumes of routine samples. The sensitivity of this rapid test is poor, and improvements are needed to enhance its performance.

Keywords: Antigen testing; COVID-19; Point-of-care; Rapid diagnosis; SARS-CoV-2; qRT-PCR.

Fig. 1

Evolution of the number of positive and negative PCRs among samples sent for confirmation of negative antigenic testing and the percentage of positive PCRs (solid line) and antigenic testing (dashed line) during the 4 weeks of observation.