LAUSR

Diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in patients with suspected coronavirus disease 2019 (COVID-19) is most widely performed with real time polymerase chain reaction (RT-PCR) considered as gold standard [1]. RT-PCR is a highly sensitive diagnostic method for detecting viral ribonucleic acid (RNA) with the disadvantages of logistics for transport of samples to specific laboratories and the relative long duration of the diagnostic method. Positive SARS-CoV-2 RT-PCR also do not allow definitive conclusions whether the subject is still contagious or not. This can be partly attained by establishing the cycle threshold (Ct) value depicting the particular amount of viral RNA in the sample and thus allowing a conclusion on the viral load and infectivity [2]. However, because of the diagnostic effort and duration of the test, faster and less laborious tests attract interest which may help to rapidly identify and contain infected individuals. Therefore, point-of-care-testing with antigen tests, providing results in a couple of minutes, is currently evaluated for routine clinical use. We compared the SARS-CoV-2 antigen detection in nasopharyngeal swab samples by the PanbioTM COVID-19 Ag Rapid test (Abbott, Chicago, Illionis) with the simultaneous routinely conducted RT-PCR analysis of SARS-CoV-2 orf1 RNA detection with the cobas® analyzer (Roche Diagnostics GmbH, Mannheim, Germany). The nasopharyngeal swab samples were collected from 53 patients with PCRconfirmed SARS-CoV-2 infection during their hospital stay in different stages of the disease. PanbioTM COVID-19 Ag Rapid test was performed right after nasopharyngeal swab sampling while RT-PCR was routinely performed in our central laboratory facility. RT-PCR was negative in two patients suggesting an already subsided infection; consistent with it the PanbioTM COVID-19 Ag Rapid test was also negative. Among 51 RT-PCR SARS-CoV-2 positive patients, the PanbioTM COVID-19 Ag Rapid test was positive in 31 subjects depicting a poor sensitivity of 60.8% (95% CI 46.1–74.2%), compared to 93.3% in the manufacturer’s information. In the 14 patients with a Ct-value ≤ 25, being indicative for higher viral loads, the sensitivity for the PanbioTM COVID-19 Ag Rapid test was at a level of 85.7% (95% CI 57.2–98.2%, Table 1). PanbioTM COVID-19 Ag Rapid test was positive in 36.4% (95% CI 17.2–59.3%) of the patients with a Ctvalue > 30 (Table 1). Of note, when we included subjects with a Ct value ≤ 30, which is considered to be a threshold for infectivity and currently recommended by the German Robert Koch Institute as an important cutoff to identify SARS-CoV2 contagious subjects, we found that the Ag rapid test correctly identified 79.3% of individuals. However, looking on the other site of the coin we found that subjects with a Ct-value > 30 were antigen positive in 36.4% of cases. The test is still positive in a considerable number of patients which are considered as being non-infectious according to the German Robert Koch Institute. Changing the cut-off value to Ct > 33 reduced the positive results of the PanbioTM COVID-19 Ag Rapid test to 16.7% (Table 1) whereas no positive PanbioTM COVID-19 Ag Rapid test result was achieved in subjects with RT-PCR Ct-values ≥ 35 (n = 4). These results indicate a poor performance of the PanbioTM COVID-19 Ag Rapid test detecting SARS-CoV-2 antigen in patients with low virus load but a good performance in patients with high virus load. Viral load of SARSCoV-2 was reported to be the highest around the time of symptom onset, and most probably become undetectable within approximately 2 weeks. Additionally, patients with more severe symptoms also seems to have a higher viral * Günter Weiss [email protected]