Coronavirus disease 2019 (COVID-19), an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has evolved into a global pandemic and is still a major health concern around the world. Those infected with the SARS-CoV-2 experience a variety of symptoms, including fever, cough, exhaustion, shortness of breath, headache, sore throat, and loss of smell and taste [3,4,5]^[1][2][3]. The highly infectious SARS-CoV-2 is currently known to be transmitted to other people through respiratory droplets and aerosols exhibited by infected individuals [6]. 

Rapid testing of those suspected of having COVID-19 infection is critical so that those who are infected can rapidly self-isolate, reducing the risk of SARS-CoV-2 spreading into the community. The reverse transcription quantitative polymerase chain reaction (RT-qPCR), which detects SARS-CoV-2 RNA in respiratory tract samples, is presently the gold standard for COVID-19 diagnosis [13,14]^[4][5]. Nonetheless, RT-qPCR requires competent workers, expensive equipment (thermal cycler with fluorimetry) and laboratory facilities (biosafety level 3, BSL-3) for RNA extraction. This could be a challenge in resource-constrained communities, such as those lacking basic laboratory facilities. Furthermore, RT-qPCR is time-consuming, with a sample-to-result turnaround time of up to 4 h, and it may not be acceptable to use in an emergency [15]^[6]. Rapid point-of-care tests (POCTs) that are less expensive and transportable in the field are needed to improve the surveillance of SARS-CoV-2 and control the outbreak. The scientific community has achieved extraordinary progress in response to the limitations of RT-qPCR, resulting in the development of rapid diagnostic tools for improving SARS-CoV-2 surveillance. Rapid antigen tests (RATs) offer results more rapidly (approximately 15–30 min), are easy to perform and do not require highly trained staff and specialized laboratory equipment. Rapid SARS-CoV-2 antigen detection reduces the turnaround time for obtaining the results, allowing the immediate isolation of the infected individuals. As a result, the utilization of SARS-CoV-2 antigen detection will be beneficial for reducing the transmission rate. In order to control infection sources, highly effective, rapid and low-cost diagnostic screening of a vulnerable population is required. This diagnostic capability also assists policymakers in assessing when and to what extent restrictions should be relaxed to help the economy recover. While numerous studies have presented antigen-based assays for SARS-CoV-2 surveillance, it is critical to analyse and make conclusions regarding the performance of these assays and the quality of these studies in a systematic manner. Previously, two systematic reviews have evaluated the pooled sensitivity and specificity of RATs for SARS-CoV-2 detection [16,17]. However, these reviews included only studies published up to 13 January and 30 April 2021, respectively. Since more diagnostic studies involving evaluation of the performance of RATs for SARS-CoV-2 have been published recently, it is important to update the available evidence so that more comprehensive findings can be obtained. These findings will help clinicians make better decisions about how to use antigen detection with the highest diagnostic specificity and sensitivity for contact monitoring. As a result, the aim of this systematic review was to update the diagnostic performance of rapid antigen tests for SARS-CoV-2.

2. Performance of Rapid Antigen Tests for COVID-19 Diagnosis

Most of the studies still from European countries such as Germany, Spain, Italy, the Netherlands, France, Belgium and Switzerland. Lack of studies from West and Southeast Asian countries, South America and Africa, highlighting the current gap pertaining to understanding the RATs performance in such geographical areas. A similar observation was reported by the previous study, in which, most of the included studies were from Germany, Spain and Italy [17]^[7]. The plausible reason may be attributed to the fact that most kits are manufactured in European countries; thus, such test kits were easier to obtain in those countries as compared to others where supply shortages are commonly reported. The second reason is that these countries were badly affected by COVID-19 in the beginning of the outbreak. Therefore, the COVID-19 RAT is becoming popular across European countries as governments’ efforts to slow the spreading of the virus by tracking infected individuals.

Panbio™ COVID-19 Ag RDT (Abbott, Jena, Germany) and STANDARD Q COVID-19 Ag Home Test (SD Biosensor, Seoul, South Korea) were used in the majority of studies (25 and 11 studies, respectively) followed by SARS-CoV-2 Rapid Ag Test (Roche, Basel, Switzerland) and Lumipulse G SARS-CoV-2 Ag (Fujirebio, Tokyo, Japan). This could be due to the fact that Panbio™ COVID-19 Ag RDT (Abbott, Jena, Germany) and STANDARD Q COVID-19 Ag Home Test (SD Biosensor, Seoul, South Korea) are two RAD kits that are currently included under the ‘WHO Emergency Use Listing for In vitro diagnostics (IVDs) Detecting SARS-CoV-2′ [113]^[8]. There is still a lack of evaluation for newly developed test kits such as CoviNAg ELISA Kit (XEMA, Moscow, Russia) and Sienna-Clarity COVID-19 Ag RTC (Salofa Oy, Salo, Finland). Therefore, future diagnostic evaluation studies should include the newly developed test kits so that more data can be obtained for future comparative analyses.

Immunochromatography, which involves spotting antibodies onto nitrocellulose membranes that interact with specific antigens in patient samples is the basis of RATs. The antigen–antibody interaction can be visualised manually or by using an immunofluorescence machine reader. The genome of SARS-CoV-2 comprises genes the responsible for four structural proteins such the spike (S), envelope (E), membrane (M) and nucleocapsid (N) [114]^[9]. N protein is frequently employed as a target analyte in RATs for COVID-19 diagnosis. N-protein is mostly expressed during the early stages of SARS-CoV-2 infection and has the least amount of variation in its gene sequence, indicating that it is a stable protein [115]^[10].

Most studies used nasopharyngeal swabs as specimens examined for evaluating the performance of the test kits. Only a few studies used nasal swabs, saliva and throat swabs. This observation signaling the need for more studies using such specimens so that alternative sampling approaches for the rapid detection of SARS-CoV-2 could be identified. Subgroup analysis of the test kits based on specimens showed nasal swabs gave the highest sensitivity of 83% (95% CI: 80–86) followed by nasopharyngeal swabs 71% (95% CI: 70–72), throat swabs 69% (95% CI: 63–75) and saliva 68% (95% CI: 59–77). Interestingly, nasopharyngeal swabs are currently considered as the gold standard specimen for SARS-CoV-2 laboratory diagnosis. This finding may suggest that nasal swab could replace nasopharyngeal swabs as the gold standard specimen. However, it is important to note that the average sensitivity of the nasal swabs was calculated only based on 2148 samples as compared to 38548 samples for nasopharyngeal swabs. Thus, evaluation using more samples are still needed to provide a comprehensive conclusion regarding the nasal swabs’ performance. Lower sensitivity for throat swabs was in agreement with a recent study on the performances of the different sampling approaches for SARS-CoV-2, which reported lower sensitivity by throat swabs (68%) as compared to nasal swabs and saliva [116]^[11].

The sensitivity and specificity of the 30 RATs ranged from 37% to 90% and 65% to 100%, respectively, whereas the overall pooled sensitivity and specificity of RATs were 70% (95% CI: 69–71) and 98% (95% CI: 98–98), respectively (Figure 1). Based on sample size, the SARS-CoV-2 Ag Test (LumiraDx GmbH, Cologne, Germany) showed the highest sensitivity of 86% followed by the Lumipulse G SARS-CoV-2 Ag (Fujirebio, Tokyo, Japan) and BinaxNOW™ COVID-19 Ag Self-Test (Abbott, Jena, Germany) with sensitivities of 83% and 79%, respectively. The different results could be related to various study methods, RAT kits manufacturers, patient selection, types of specimens and the stage of disease at the time of sample collection. Based on the meta-analysis, the RAT kits that meet WHO criteria are Sienna-Clarity COVID-19 Ag RTC (Salofa Oy, Salo, Finland), Biosynex COVID-19 Ag BSS Test (Biosynex, Strasbourg, France), Innova Rapid SARS-CoV-2 Ag Test (Xiamen Biotime Biotechnology, Fujian, China), Dräger Ag Test SARS-CoV-2 (Dräger, Lübeck, Germany), SARS-CoV-2 Ag Test (LumiraDx GmbH, Cologne, Germany), Lumipulse G SARS-CoV-2 Ag (Fujirebio, Tokyo, Japan), S-PLEX SARS-CoV-2 N Kit (MesoScale Diagnostics, Maryland, USA) and BinaxNOW™ COVID-19 Ag Self-Test (Abbott, Jena, Germany). 

Figure 1.

The forest plot of sensitivity and specificity of rapid antigen tests for SARS-CoV-2

According to the WHO, RATs should be prioritized for use in symptomatic individuals who meet the COVID-19 case definition, as well as to test asymptomatic individuals at high risk of infection, particularly in settings where NAAT testing capacity is limited. RATs showed similar specificity (98%) for symptomatic and asymptomatic patients. However, the sensitivity of RATs for symptomatic patients was greater than asymptomatic patients (82% vs. 68%). A similar finding was reported by the previous systematic reviewudies, in which, the sensitivity of RATs is higher when used for symptomatic patients [16,17]^[12][7]. In addition, there is a clear association between Ct values of RT-qPCR and RATs’ sensitivity and specificity. The lower the Ct value (≤25), the greater the sensitivity and specificity of RATs, whereas the higher the Ct value (<25), the lower the sensitivity and specificity of RATs. Ct values, on the other hand, cannot be directly compared between tests and must be interpreted with caution because they are impacted by sample type, sample collection timing, and assay design [118]^[13].

The severity of the disease, the timing of sample collection, the types of samples, and sample handling techniques all influence antigen levels in samples [20]^[14]. It is hard to determine if the difference in observed sensitivity is due to the test’s performance or the qualities of the samples utilized in the test without this information. Unfortunately, the majority of the studies did not provide information on antigen levels in the samples. Information about disease severity and sample collection timing are often missing. Future research should include this information to enable for a more accurate assessment of diagnosis test performance as well as the identification of their actual limitations.

53. Conclusions

Future study should attempt to perform double-blinded index test as such improvement in study design would reduce index test risk of bias. CoviNAg ELISA Kit (XEMA, Moscow, Russia) and Sienna-Clarity COVID-19 Ag RTC (Salofa Oy, Salo, Finland) had the highest diagnostic sensitivity among all RATs, while MEDsan SARS-Cov-2 Ag (MEDsan GmbH, Hamburg, Germany) had the lowest diagnostic sensitivity. More studies from the Middle East and Southeast Asian, South American, and African countries are warranted so that a comprehensive subgroup analysis based on regions can be performed. Nasal swabs showed the highest sensitivity followed by nasopharyngeal swabs, throat swabs and saliva. The RATs showed higher sensitivity for those patients with symptoms and Ct value ≤25. Comparative performance of the RATs using less invasive sampling approaches is still lacking. Improvement in these key areas would help to boost acceptability and accessibility for large practical of RATs for rapid surveillance of SARS-CoV-2, allowing immediate isolation of the infected individuals and reducing the disease’s transmission.