Challenges in the Detection of SARS-CoV-2: Evolution of the Lateral Flow Immunoassay as a Valuable Tool for Viral Diagnosis

Abstract

SARS-CoV-2 is an emerging infectious disease of zoonotic origin that caused the coronavirus disease in late 2019 and triggered a pandemic that has severely affected human health and caused millions of deaths. Early and massive diagnosis of SARS-CoV-2 infected patients is the key to preventing the spread of the virus and controlling the outbreak. Lateral flow immunoassays (LFIA) are the simplest biosensors. These devices are clinical diagnostic tools that can detect various analytes, including viruses and antibodies, with high sensitivity and specificity. This review summarizes the advantages, limitations, and evolution of LFIA during the SARS-CoV-2 pandemic and the challenges of improving these diagnostic devices.

Keywords: COVID-19; SARS-CoV-2; antibodies; biosensors; diagnosis; lateral flow immunoassay.

Figure 1

Principle of LFIA test. LFIA test detects the target molecule on an absorbent membrane with antibodies aligned to form the test and control lines. The sample is placed on a sample pad, then migrates to the conjugate pad, which contains the immobilized conjugate, usually made of nanoparticles (colloidal gold, colored or fluorescent latex, colored cellulose) conjugated to antibodies or antigens. The sample interacts with the conjugate, and both migrate to the next section of the strip, where the biological components of the assay (proteins/antibodies/antigens) are immobilized. In this section, the analyte and conjugate are captured. Excess reagent passes through the capture lines and accumulates on the absorbent pad. The results are interpreted on the nitrocellulose membrane as the presence or absence of the test and control lines.

Figure 2

Main methods for detecting SARS-CoV-2. Currently, the detection of SARS-CoV-2 is carried out using molecular tests and biosensors. RT-PCR is the gold standard for detecting SARS-CoV-2 viral RNA. There are other available molecular tests used based on RT-LAMP. Biosensors with transducers have been developed to detect viral antigens or antibodies. LFIA is the representative test for mass diagnosis of SARS-CoV-2 in POC and antibody prevalence studies worldwide. These biosensors have been widely marketed because they can be applied at the POC and are inexpensive, fast, and easy to read.

Figure 3

LFIAs detect IgG and/or IgM antibodies in asymptomatic, symptomatic, and immunized individuals. (A) LFIAs with qualitative readouts are available with the SARS-CoV-2 NP antigenic protein and the S protein. (B) LFIAs coupled to portable spectrophotometer have a greater sensitivity for evaluating the humoral response of convalescent, symptomatic, asymptomatic, and immunized individuals. The graphs on the right indicate the interpretation of LFIA results showing the humoral response in SARS-CoV-2 infected and uninfected individuals and those who have received at least the first dose of vaccine.

Figure 4

Strategies to improve the detection of SARS-CoV-2 antigens or nucleic acids by Lateral flow immunoassay. (A) The sensitivity of LFIAs focuses on developing high-affinity antibodies for the antigen. (B) Use of nanomaterials as markers that potentiate the antigen–antibody signal. (C) The detection of nucleic acids, incorporating different isothermal amplification techniques.