key: cord-1023334-wdr6nq40
authors: Yen, Yung-Feng; Hu, Hsiao-Yun; Chen, Chu-Chieh; Chu, Dachen; Lee, Ya-Ling
title: Clinical sensitivity of rapid antigen test during a COVID-19 outbreak in Taipei, May to June 2021
date: 2022-03-28
journal: J Formos Med Assoc
DOI: 10.1016/j.jfma.2022.03.016
sha: 0997b97d58b0b494b08196a45af86170ad74c48e
doc_id: 1023334
cord_uid: wdr6nq40

Background/Purpose This population-based study aimed to compare the accuracy of Rapid antigen detection (RAD) and reverse transcription-polymerase chain reaction (RT-PCR) assays for diagnosing individuals infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) during the COVID-19 outbreak in Taipei, from May to June 2021. Methods In response to the outbreak of COVID-19 in mid-May 2021, Taipei City Hospital set up 12 citywide proactive community testing (PCT) stations for early identification of infected individuals from May 17 to June 20, 2021. Individuals with RAD positivity were isolated and later confirmed by RT-PCR. The c-statistic value was estimated to indicate the level of diagnostic accuracy of RAD tests. Results Of the 33,798 individuals who were evaluated for SARS-CoV-2 infection, 4.4% tested positive for RAD. There was a moderate concordance (kappa = 0.67) between the RAD tests and RT-PCR assay for identifying infectious individuals. The c-statistic value of the RAD test for the diagnosis of SARS-CoV-2 infection was 0.8. There was a positive linear trend between the accuracy of the RAD tests and the prevalence of SARS-CoV-2 infection in the study population (β = 0.04; p=0.03). As the cycle threshold value decreased, the sensitivity rate of the RAD tests increased (p<.001). After implementation of the PCT program, the prevalence of COVID-19 decreased from 8.4% to 3.3% (p<.001). Conclusion Proactive community testing for SARS-CoV-2 infection using RAD tests could rapidly identify and quarantine the most infectious patients in the early phase of COVID-19 outbreak.

The coronavirus disease 2019 , caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been spreading worldwide since January 2020. As of January 3, 2022, 281.8 million individuals have been infected with SARS-CoV-2, with the number of deaths reaching 5.4 million. 1 SARS-CoV-2 is highly contagious and thus susceptible to outbreaks. Early identification of patients with COVID-19 is essential to control the outbreak. 2 A previous report indicated that proactive testing for SARS-CoV-2 in a population could identify symptomatic and asymptomatic infectious individuals early and prevent the onward infection of others. 3 World Health Organization (WHO) also recommended extensive testing as an important strategy to control the increasing incidence of COVID-19 during the pandemic. 4 The current standard test for laboratory diagnosis of SARS-CoV-2 infection is the real-time reverse transcription-polymerase chain reaction (RT-PCR) assay. 5 Although the results of the RT-PCR assay are normally available within four hours, some reports showed that the results of the RT-PCR assay could be delayed by three to seven days. 6 A long waiting period to obtain results increases the risk of virus transmission and causes more challenges to controlling the COVID-19 outbreak.

Rapid antigen detection (RAD) tests are an alternative assay for diagnosing active J o u r n a l P r e -p r o o f infection by detecting SARS-CoV-2 viral antigens. The test results of RAD can be interpreted without specialized instruments and are available within 15-20 mins. 9 However, in contrast to the analytical sensitivity (positive rate to detect standard laboratory SARS-CoV-2 samples) required for FDA licensure, the public health usefulness of RAD tests actually depends on their clinical sensitivity (positive rate to detect all patients with SARS-CoV-2 infection, including those in latency period), 7 Taipei City government adopted a new quarantine policy based on a single RAD test. 14 Individuals who tested positive on RAD were isolated and their results were later confirmed by RT-PCR during the COVID-19 outbreak in May, 2021. 14 When the community is threatened by the spread of COVID-19, the most important measure is the early identification and isolation of individuals infected with the disease. 2 Previous studies found that RAD tests are an important tool in assisting the control of COVID-19 outbreaks. 9, 15 However, previous reports were unable to evaluate the accuracy of RAD tests for the diagnosis of SARS-CoV-2 during the different stages of the COVID-19 outbreak. 9, 15 Moreover, it remains unclear whether RAD tests are actually useful in identifying the most infectious patients during a COVID-19 outbreak, and thus facilitate early quarantine and contact tracing of the suspected infectious individuals. Therefore, we conducted a population-based study to compare the accuracy of RAD and RT-PCR assays in diagnosing individuals infected with SARS-CoV-2 during the COVID-19 outbreak in Taipei, from May to June 2021.

In mid-May 2021, Taiwan had its first huge outbreak of SARS-CoV-2, which was particularly severe in Taipei. 13 Taipei City Hospital (TCH), a healthcare organization affiliated with the Taipei City Government, set up 12 extensive proactive community testing (PCT) stations to identify and isolate individuals infected with SARS-CoV-2.

People who had been in contact with a patient with COVID-19 were informed by the Taipei Bureau of Public Health to receive free-of-charge RAD tests and RT-PCR assays. The RAD test at the PCT stations was also conducted for those who had symptoms of COVID-19. When the RAD test in an individual showed positivity for SARS-COV-2 infection, the patient was admitted to the designated isolation centers in Taipei.

This study recruited individuals who were tested for SARS-COV-2 infection at various Taipei PCT stations from May 17 to June 20, 2021, using RAD and RT-PCR assays. During the RAD tests, participants' demographic data (e.g., age and sex) were collected. Individuals who did not complete all of the demographic questions were excluded from the analysis. This study was approved by the Institutional 

At the time of the RAD tests, respiratory samples, mainly nasopharyngeal and throat swabs, were collected. Samples in study individuals were mixed in 2 mL of viral transport media, comprising Hanks' balanced salt, 0.4% fetal bovine serum, and HEPES, as well as antifungal and antibiotic agents. Samples were transported at 2-8 °C to the Microbiology Laboratory, Taiwan CDC and then were processed within a few hours. All samples were processed in biosafety level-2 enhanced (BSL-2 +) and biosafety level-3 (BSL-3) facilities with proper personal protective equipments.

The RAD tests in this study were rapid chromatographic immunoassays for the detection of SARS-CoV-2 nucleocapsid (N) antigens in respiratory specimens. 16 The RAD test device had two precoated lines on the result window: test (T) and control (C) lines. The test (T) region was coated with mouse monoclonal anti-SARS-CoV-2 antibody against SARS-CoV-2 N antigen, and the control (C) region was coated J o u r n a l P r e -p r o o f with mouse monoclonal anti-chicken Igγ antibody. For COVID-19 antigen testing positivity, two colored lines of test (T) and control (C) lines are presented.

At the time of the RAD tests, participants also received an RT-PCR assay to confirm SARS-CoV-2 infection. After sampling, the oropharyngeal swabs were placed in a viral transport medium for RNA extraction. We used an RNA purification kit (QIAmp Viral RNA Mini Kit, Qiagen, Germany) to extract the viral RNA. RT-PCR was performed by amplifying the RNA-dependent RNA polymerase gene (RdRp), envelope (E), and nucleocapsid (N) genes. 17 The E gene and N gene assays were used as first-line screening targets, which was then followed by confirmatory testing with the RdRp gene assay.

First, we analyzed the demographic data of the study individuals. Then we presented the continuous data as the mean (standard deviation [SD]), and used a two-sample t-test to compare groups. We also used Pearson's χ 2 test to analyze the categorical data , where appropriate.

The concordance between the RAD test and the RT-PCR in terms of diagnosing SARS-CoV-2 infection was calculated as the overall percentage agreement using 2 × 2 contingency tables. The strength of this agreement was assessed using the kappa statistics. 18 To assess the diagnostic accuracy of RAD tests, we calculated the sensitivity (the ability to identify individuals infected with SARS-CoV-2), specificity (the ability to identify those not infected with SARS-CoV-2), positive predictive value (PPV) (the proportion of individuals who were infected with SARS-CoV-2 when the RAD tests were positive), and negative predictive value (NPV) (the proportion of individuals who were not infected with SARS-CoV-2 when the RAD tests were negative). 19 The c-statistic value, also known as the area under the curve, was estimated to indicate the diagnostic accuracy of RAD tests. 20 Table 1 shows the characteristics of the study participants with positive and negative RAD tests. Compared to individuals with RAD test negativity, those with RAD test positivity were older. In terms of the RT-PCR assay, individuals with RAD test positivity were more likely to show RT-PCR positivity. Table 2 shows the kappa statistics between the RAD and RT-PCR assays in terms of diagnosing SARS-CoV-2 infection. A moderate concordance (kappa = 0.67) was found between RAD tests and the RT-PCR assay to identify individuals with COVID-19 infection. Participants with both RAD test negativity and RT-PCR J o u r n a l P r e -p r o o f positivity were the major contributors to the discrepancy between these two tests. Table 3 shows the accuracy of RAD tests for the diagnosis of SARS-CoV-2 infection.

The c-statistic value of the RAD test for the diagnosis of SARS-CoV-2 infection was 0.8. There was a linear trend between the accuracy of the RAD tests and the prevalence of SARS-CoV-2 infection in the study participants (β = 0.04; p=0.03). (Table 4 ). There was a negative association between the sensitivity of the RAD tests and the Ct value in the study participants (p<.001). This study revealed that the RAD tests had good accuracy for the diagnosis of SARS-CoV-2 infection during the early phases of the COVID-19 outbreak. Despite RAD tests not being the "gold standard" when diagnosing SARS-CoV-2, they could provide results within 15-20 mins, 12 thus facilitating early quarantine of infectious individuals. Moreover, the RAD test results can be interpreted without specialized instruments, 9 whereas RT-PCR assays require laboratory facilities with robust infrastructure and a highly trained staff. 5 Our findings suggest that RAD tests could be strategic in identifying and isolating the most infectious individuals during the early phases of a COVID-19 outbreak.

This study showed a significant decrease in the accuracy of the RAD tests for the diagnosis of SARS-CoV-2 infection in the later phase of eliminating the virus from communities. Although the RAD tests could show the results quickly, 12 the sensitivity and positive predictive values (PPVs) of the RAD tests vary according to the prevalence of SARS-CoV-2 and the level of Ct value in the screening population. 10, 12 A recent meta-analysis reported that, at 5% prevalence of SARS-CoV-2 infection in symptomatic individuals, the PPVs of the RAD tests ranged from 84% to 90%, which J o u r n a l P r e -p r o o f means that between 1 in 10 and 1 in 6 positive results will be a false positive. 10 However, when the prevalence of SARS-CoV-2 infection decreased to 0.5% in the screening population, the PPVs of the RAD tests were between 11% and 28%, which means that between 7 in 10 and 9 in 10 positive results will be false positives. 10 Our study found that RAD tests had the high sensitivity and PPVs at the 8. The accuracy of the RAD tests also varies according to the level of Ct value in the screening population. A previous report showed that the sensitivity of RAD tests ranged from 92.3% to 97.8% in patients with the Ct value <25, while the sensitivity of RAD tests was between 35.6% and 65.8% in patients with the Ct value ≥25. 12 Our study found that the lower the Ct value, the higher the sensitivity rate of the RAD tests. 

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The authors are grateful to the members of the Research Office for Health Data, Department of Education and Research, Taipei City Hospital, Taiwan, for their valuable contributions to data management and statistical analysis.Funding/Support: This study was not funded.

Author disclosures: All authors reported no conflict of interest.J o u r n a l P r e -p r o o f