Performance of a Point-of-Care Test for the Rapid Detection of SARS-CoV-2 Antigen

Abstract

The rapid detection of infections caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is necessary in the ongoing pandemic. Antigen-specific point-of-care tests (POCT) may be useful for this purpose. Here, such a POCT (SARS-CoV-2 NADAL^® COVID-19 Ag) was compared to a laboratory-developed triplex real-time polymerase chain reaction (RT-PCR) designed for the detection of viral nucleoprotein gene and two control targets. This RT-PCR served as a reference to investigate POCT sensitivity by re-testing upper respiratory tract (URT) samples (n = 124) exhibiting different SARS-CoV-2 loads in terms of RT-PCR threshold cycle (Ct) values. The optical intensities of the antigen bands were compared to the Ct values of the RT-PCR. The infectivity of various virus loads was estimated by inoculating Vero cells with URT samples (n = 64, Ct 17-34). POCT sensitivity varied from 100% (Ct < 25) to 73.1% (Ct ≤ 30); higher SARS-CoV-2 loads correlated with higher band intensities. All samples with a Ct > 30 were negative; among SARS-CoV-2 free samples (n = 10) no false-positives were detected. A head-to-head comparison with another POCT (Abbott, Panbio™ COVID-19 Ag Rapid Test) yielded similar results. Isolation of SARS-CoV-2 in cell-culture was successful up to a Ct value of 29. The POCT reliably detects high SARS-CoV-2 loads and rapidly identifies infectious individuals.

Keywords: COVID-19; PCR; POCT; antigen; comparison; diagnostic.

Figure 1

Detection of SARS-CoV-2 antigen in samples of the upper respiratory tract (n = 124) by the NADAL^® COVID-19 Ag test (Nal von Minden). The detection rate was assessed in terms of the threshold cycle (Ct) of the N-gene triplex RT-PCR. The raw data are listed in the table below the figure.

Figure 2

Optical intensity of the SARS-CoV-2 antigen band observed in the NADAL^® COVID-19 Ag test (Nal von Minden). The intensity was measured automatically by scanning and rated in relation to the threshold cycle (Ct) of the N-gene triplex RT-PCR. Mean intensities and standard deviations are given. The samples correspond to those in Figure 1.

Figure 3

Head-to-head comparison of two POCTs for detection of SARS-CoV-2 antigen. Upper respiratory tract samples which have been already tested in the NADAL^® COVID-19 Ag test (Nal von Minden) were re-tested with the Panbio™ COVID-19 Antigen rapid test (Abbott, n = 21). The raw data are listed in the table below the figure.

Figure 4

Isolation of SARS-CoV-2 from upper respiratory tract (URT) samples in Vero cells. A successful isolation in cell culture was assumed if the supernatant of the corresponding well had a threshold cycle (Ct) < 20 in the N-gene triplex RT-PCR. The isolation rate was estimated as a function of viral load by mean of the Ct (N-gene triplex RT-PCR) of the original URT sample. The results after the first round on Vero cells and after the third round are shown. Four positive and negative controls (pc, nc) were included. The table below the figure lists the result per URT sample and the corresponding Ct. The cytopathic effect observed after the third round in Vero cells is documented in Figure 5.

Figure 5

Cytopathic effect after two passages (third round) of 64 upper respiratory tract samples (no. 9–72) in Vero cells with respect to the threshold cycle (Ct) values of the original sample (N-gene triplex RT-PCR). For comparison, positive controls (vc, no. 73–76; own SARS-CoV-2 isolates), negative controls (nc, no. 5–8; SARS-CoV-2 free patient samples) and cell controls (cc, no. 1–4, and ni; cells in medium) are included. Note that the destroyed cell layer in no. 31 represents an artifact and does not correlate to the result of N-gene triplex RT-PCR from the supernatant (compare Figure 4).