Rapid antigen tests for the detection of SARS-CoV-2: A narrative review

The rapid identificationand isolation of COVID-19 patients has become the cornerstone for the control of the recent outbreak. Real-time quantitative polymerase chain reaction is routinely used to confirm COVID-19 diagnosis and is considered the gold standard due to high sensitivity and specificity. Nevertheless, it usually takes several days and a relatively higher cost. Antigen tests based have emerged to cope with such disadvantages, by offering rapid results, an easy-to-use procedure, and low costs. The objective of the narrative review was to provide up-to-date data about CE-marked rapid antigen tests (RATs) for COVID-19. Given their large number, the study only focused on representative and widely used in Spain (Standard Q, Nadal, Panbio, CerTest, and Wondfo). RATs have become a very useful and validated tool for controlling the spread of COVID-19 allowing the rapid identification of active infection and isolation of positive patients. The present revision of the literature has demonstrated that sensitivity and specificity of all available RATs in Spain are high and accomplish European regulations and WHO recommendations.


Introduction
Coronaviruses are an important family of pathogenic agents that affect the human respiratory system, and were responsible for worldwide outbreaks such as the severe acute respiratory syndrome (SARS)-CoV in 2003, or the Middle East respiratory syndrome (MERS)-CoV in 2012. 1,2 By December 2019, a cluster of atypical pneumonia cases were reported in Wuhan (China). The etiological agent was identified as a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and its related disease was named as coronavirus disease 2019 . 3 The disease shows diverse clinical manifestations, i.e. from no or mild symptoms to severe pneumonia with multi-organ failure, including acute respiratory distress syndrome, sepsis, and septic shock. 4 On 30 th January 2020 the World Health Organization (WHO) declared COVID-19 outbreak to be a public health emergency of international concern; 5 the sixth after H1N1 (2009), Polio (2014), Ebola in West Africa (2014), Zika (2016) and Ebola in the Democratic Republic of Congo (2019). 6 The first case of COVID-19 in Spain was confirmed on 31 st January 2020. 7 Since then, a total of 3,096,343 individuals have been tested positive, and 65,979 ones have passed away. 8 The rapid identification and isolation of positive patients has become the cornerstone for the control of the COVID-19 outbreak. 9 Real-time quantitative polymerase chain reaction (RT-qPCR), with nasopharyngeal swabs, throat swabs, or saliva samples, is routinely used to confirm the COVID-19 diagnosis. 10 It is considered the gold standard due to its high sensitivity and specificity. 11 Nevertheless, RT-qPCR testing usually takes several days and relatively higher costs to be completed. It requires the transportation of samples from the place of collection to the specialized laboratory, 4-6 hours of methodology, limited laboratory capacity (trained staff, special equipment, reagents, disposables, etc.), and high sample volumes. 11 Antigen tests based on lateral flow immunoassays have emerged to cope with such a disadvantage scenario, by offering rapid results (10-30 minutes), through an easy-to-use procedure, and low costs. By December 2020, the European Commission purchased over 20 million rapid antigen tests (RATs) J o u r n a l P r e -p r o o f for European members, and stated recommendations for their use and validation. 12 The objective of the present narrative review was to provide up-to-date data about RATs for COVID-19 with European conformity (CE marked). Given their large number, 13 the study only focused on representative and widely used in Spain, i.e. Standard Q (SD Biosensor Inc., Republic of Korea; F. Hoffmann-La Roche Ltd, Switzerland), Nadal (Nal von minden GmbH, Germany), Panbio (Abbott Laboratories GmbH, Germany), CerTest (Certest Biotec, S.L., Spain), and Wondfo (Guangzhou Wondfo Biotech Co. Ltd, China).

Methods
A systematic review of available studies involving RATs was carried out in PubMed database.
Keywords included: "antigen test", "COVID-19", "SARS-CoV-2", "Standard Q", "Roche", "Nadal", "Panbio", "Abbott", "CerTest", and "Wondfo". The date of search was February 18 th 2021. Only studies providing information about sensitivity and specificity of RATs were analyzed. With the aim of including more studies (especially preprint papers, studies that have not been published on PubMed, or information from manufacturers), manual searches were carried out in Google by using the same keywords.

Standard Q COVID-19 Antigen Test
The Standard Q test was the second commercialized RAT in Spain. It detects the presence of identified. Thus, overall sensitivity, specificity, PPV, and NPV were 70.6%, 100.0%, 100.0%, and 87.4%, respectively.

Nadal COVID-19 Antigen Test
Nadal antigen test is another RAT widely used in Spain. 26 The test directly checks for the presence of specific SARS-CoV-2 nucleocapsid proteins in 15 minutes. According to the manufacturer's information, the diagnostic sensitivity is 80.2% (95%CI: 73.9-85.3%) for a Ct value 20-37 or 97.6% (95%CI: 93.1-99.2%; Table 2) if Ct value 20-30, and the diagnostic specificity is more than 99.9% (95%CI: 97.7-100.0). 26 These data derive from a study with 348 samples, 46.3% being RT-qPCR positive, and 43.1% Nadal test-positive. No false positives with Nadal test were reported. Few studies have been published to evaluate the Nadal antigen test for the diagnosis of SARS-CoV-2 infection. All of them have compared the analytical sensitivity, and clinical sensitivity and specificity in diverse RATs (Table 3). 18

Panbio COVID-19 Antigen Rapid Test
Panbio was the first commercialized RAT for COVID-19 in Spain. 29 The test detects the presence of a SARS-CoV-2 nucleocapsid protein in nasal or nasopharyngeal swab specimens within 15 minutes. 30 (Table 3). 31

CerTest SARS-CoV-2 Card Test
CerTest test represents the first commercialized RAT that is manufactured by a Spanish Biotech company. 42 It is also based on the qualitative detection of SARS-CoV-2 membrane proteins from nasopharyngeal swab samples. The only available information related to its clinical sensitivity and specificity derives from the leaflet of the manufacturer. 43

Wondfo 2019-nCoV Antigen Test
Wondfo test is another commercialized and widely used RAT in Spain. It qualitatively detects a SARS-CoV-2 nucleocapsid protein in nasopharyngeal/oropharyngeal samples within 15-20 minutes. 44 Data on sensitivity and specificity also derive from the manufacturer and a couple of reports from FIND foundation (Table 4). 45

Strengths and limitations on rapid antigen tests
Overall clinical sensitivity of RATs has demonstrated to be high; however there is some variability among studies. By contrast, overall specificity is consistently elevated in all of them.
RATs are especially adequate in patients with high viral loads (Ct values ≤25 or >106 genomic virus copies/mL) which frequently present symptoms within the first 5-7 days of the disease. 48 However, symptomatic patients with onset of symptoms beyond 7 days are more likely to show lower viral loads, and thus the likelihood of false negative results with RATs is greater. According to a recent systematic review and meta-analysis with data from 52 studies that evaluated the accuracy of 20 different RATs, sensitivity and specificity are 73.8% (95%CI: 68.6-78.5) and 99.7% (95% CI: 99.3. 99.9), respectively. 49 The WHO established the minimum requirements of sensitivity (≥80%) and specificity (≥97%) for RATs to be used for the diagnosis of SARS-CoV-2 infection when RT-qPCR is unavailable or where prolonged turnaround times preclude clinical utility. 48 Available information on sensitivity and specificity from RATs are relatively scarce and derive from studies with differential designs, participating subjects, and test brands being evaluated. Lateral flow assays are point-of-care formats, and involve simple and easy-to-produce devices. 50 RATs can also provide a result in minutes, in contrast to the few days required with RT-qPCR.