Rapid Antigen Diagnostics as Frontline Testing in the COVID-19 Pandemic

Abstract

The ongoing global COVID-19 pandemic, caused by the SARS-CoV-2 virus, has resulted in significant loss of life since December 2019. Timely and precise virus detection has been proven as an effective solution to reduce the spread of the virus and to track the epidemic. Rapid antigen diagnostics has played a significant role in the frontline of COVID-19 testing because of its convenience, low cost, and high accuracy. Herein, different types of recently innovated in-lab and commercial antigen diagnostic technologies with emphasis on the strengths and limitations of these technologies including the limit of detection, sensitivity, specificity, affordability, and usability are systematically reviewed. The perspectives of assay development are looked into.

Keywords: COVID-19; antigen tests; immnuoassays; in vitro diagnostics.

Figure 1

Timeline of the emergence of SARS‐CoV‐2 variants and those of greater concern.^{[ 116 ]} a) Timeline of the emergence of SARS‐CoV‐2 variants. All variants are presented with corresponding first‐identified date, pango lineage, and first‐identified location, ten of which (labeled in orange) have been named by WHO using letters of the Greek Alphabet as of December 2021. b) Date of designation of variants of concern by WHO.

Figure 2

Estimated viral load and immune responses before and after symptom onset^[ 106, 117, 118, 119 ^] and appropriate diagnostic methods at different phases of infection. This figure was created with BioRender.com.

Figure 3

Mutations of S proteins in different SARS‐CoV‐2 variants. Mutations are called in reference to Wuhan‐Hu‐1 (NCBI Reference Sequence: NC 045512.2).^[ 120, 121 ^] S1 and S2 subunits are labeled in blue and red, respectively.

Figure 4

Common SARS‐CoV‐2 viral antigen diagnostic methods based on different mechanisms. This figure was created with BioRender.com.

Figure 5

Schematic of testing workflow and mechanism of a typical LFA, created with BioRender.com.

Figure 6

Basic setup and procedures of sandwich ELISA and principles of direct, indirect, and competitive ELISAs. This figure was created with BioRender.com.

Figure 7

Mechanistic illustration of CLIA detecting SARS‐CoV‐2 antigen. This figure was created with BioRender.com.

Figure 8

Principles of electrochemical techniques used for the detection of SARS‐CoV‐2 viral antigens. Inputs of antigen–antibody binding can be output in forms of voltametric, impedimetric, and amperometric signals via an electrochemical analyzer. This figure was created with BioRender.com.

Figure 9

SPR setup for detection of COVID‐19 antigen. a) Schematic of SPR workflows. The antigens of interest are captured by antibodies bound to the gold substrate and can be detected by the change of laser signals. b) Sensorgram of an SPR measurement. Binding of antigen and antibody triggers shifts of laser intensity and resonance signal. This figure was created with BioRender.com.

Figure 10

Comparison of current antigen test kits with EUAs in the U.S. using a five‐star chart model. LFA, ELISA, CLIA, and SPR assay are expressed in blue, orange, green, purple, and pink colors, respectively. The unit of LoD is viral copies mL⁻¹. Note: These five‐star dimensions are qualitative or semiquantitative statistics of each test principle obtained from the U.S. FDA website^{[ 122 ]} and might vary with specific commercial products.

Figure 11

Statistics of COVID‐19 antigen tests issued with CE marking in European Economic Area as of April 6, 2022. All data were collected from the COVID‐19 In Vitro Diagnostic Devices and Test Methods Database.^{[ 123 ]} a) Total number of antigen test kits. b) Total number of antigen test kits classified as self‐test. c) Number of various kits categorized by the European Commission based on detection principle. d) Number of various kits categorized by the European Commission based on target. e) Number of various kits categorized by the European Commission based on specimen. f) Number of various kits categorized by the European Commission based on test format. Note: Statistical classifications in (d–f) are not exclusive as some test kits might apply to multiple targets, specimen, or test formats.