An update on lateral flow immunoassay for the rapid detection of SARS-CoV-2 antibodies

SARS-CoV-2 neutralizing antibody determination after vaccination using spectrophotometric measurement of lateral flow immunochromatography

Neutralizing antibody titers have been found to be strongly correlated with observed vaccine effectiveness against symptomatic and severe COVID-19. Few non-high complexity assays are currently available to detect the presence of neutralizing antibodies. This retrospective single-center cross-sectional study compared the performance of a lateral flow immunochromatography assay coupled with a spectrophotometric measurement system for detecting SARS-CoV-2 neutralizing antibodies against an enzyme-linked immunosorbent assay (ELISA) neutralization antibody assay in the context of post-vaccination responses. The limit of detection was similar to the ELISA with strong linearity throughout the measuring interval. Repeatability, interfering substances, and cross-reactivity studies were found to be robust. Results for 274 plasma samples on whom SARS-CoV-2 RNA test and vaccination status, including vaccination number and manufacturer, was known showed a positive predictive value (PPV) of 99.0% (CI 96.4-99.7%) and a negative predictive value (NPV) of 91.9% (CI 83.4-96.2%) compared to ELISA. The PPV for all vaccination number and manufacturer subgroups was > 95% except for those individuals who had only 1 Pfizer vaccination (PPV of 80%). The NPV for those who were PCR positive with no vaccinations was 100% while only 88.1% for those without a previous positive test or vaccination. The NPV for those with Pfizer vaccinations was 80% in contrast to 100% for those with Moderna vaccinations. Alternative methodologies requiring less sophisticated laboratory support to measure neutralizing antibodies may be useful to measure vaccine responses.

Crystal Structures of Antigen-Binding Fragment of Anti-Osteocalcin Antibody KTM219

Osteocalcin is a useful biomarker for bone formation and bone-related diseases. KTM219 is an anti-osteocalcin C-terminal peptide antibody. The single-chain variable region (scFv) and antigen-binding fragment (Fab) of KTM219 are applicable to the Quenchbody (Q-body) immunoassay. Q-body is a new type of fluorescent immunosensor, which is scFv or Fab labeled with a fluorescent dye. When Q-body binds to its antigen, the fluorescence intensity increases. The highly sensitive detection of antigens by changes in fluorescence intensity is performed in a single step by mixing the sample and reagent. In this study, to reveal the recognition mechanism of the KTM219 antibody and to discuss the structural basis for Q-body, we solved the crystal structures of Fab of the anti-osteocalcin antibody KTM219 and its complex with the antigen osteocalcin C-terminal peptide (BGP-C7). Also, we solved the structure of a KTM219 Fab crystal grown in the presence of a fluorescent dye, carboxytetramethylrhodamine (TAMRA); however, tightly bound TAMRA was not found in the electron density map. We predicted the binding sites of TAMRA in the antigen-binding pocket by docking simulations. These results support the proposed Q-body mechanism. The crystal structures of KTM219 Fab would be useful for further development and improvement of Q-body fluorescent immunosensors.

Ultrasensitive Rapid Antigen Test by Geometric Lateral Flow Assays and Highly Doped Upconversion Nanoparticles

The paper-based lateral flow assay (LFA) testing strips are currently the most widely used for point-of-care testing (POCT), valued for their rapid result turnaround times in a few minutes. However, their sensitivity has been limited. Upconversion nanoparticles (UCNPs), especially highly doped ones, have emerged as promising luminescent reporters to enhance the LFA sensitivity. These UCNPs exhibit a nonlinear enhancement in luminescence with excitation power density, necessitating higher power densities for higher brightness. In this study, we utilized a geometric paper strip design to minimize the immune reaction area and maximize the excitation power density, enabling ultrasensitive detection of the SARS-CoV-2 nucleoprotein antigen. This design also slowed the antigen flow on the paper strip, extending the reaction time between antigen and antibody, thereby enhancing the efficiency of the immune reaction. Through this design, our approach achieved over a 100-fold enhancement in the limit of detection (LOD) compared with the widely used LFAs, based on gold colloidal nanoparticles and europium nanoparticles. This innovation expands the scope of LFA applications that require a low LOD.

Optical lateral flow assays in early diagnosis of SARS-CoV-2 infection

So far, the 2019 novel coronavirus (COVID-19) is spreading widely worldwide. The early diagnosis of infection by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is essential to provide timely treatment and prevent its further spread. Lateral flow assays (LFAs) have the advantages of rapid detection, simple operation, low cost, ease of mass production, and no need for special devices and professional operators, which make them suitable for self-testing at home. This review focuses on the early diagnosis of SARS-CoV-2 infection based on optical LFAs including colorimetric, fluorescent (FL), chemiluminescent (CL), and surface-enhanced Raman scattering (SERS) LFAs for the detection of SARS-CoV-2 antigens and nucleic acids. The types of recognition components, detection modes used for antigen detection, labels employed in different optical LFAs, and strategies to improve the detection sensitivity of LFAs were reviewed. Meanwhile, LFAs coupled with different nucleic acid amplification techniques and CRISPR-Cas systems for the detection of SARS-CoV-2 nucleic acids were summarized. We hope this review provides research mentalities for developing highly sensitive LFAs that can be used in home self-testing for the early diagnosis of SARS-CoV-2 infection.

COVID-19 point-of-care tests can identify low-antibody individuals: In-depth immunoanalysis of boosting benefits in a healthy cohort

The recommended COVID-19 booster vaccine uptake is low. At-home lateral flow assay (LFA) antigen tests are widely accepted for detecting infection during the pandemic. Here, we present the feasibility and potential benefits of using LFA-based antibody tests as a means for individuals to detect inadequate immunity and make informed decisions about COVID-19 booster immunization. In a health care provider cohort, we investigated the changes in the breadth and depth of humoral and T cell immune responses following mRNA vaccination and boosting in LFA-positive and LFA-negative antibody groups. We show that negative LFA antibody tests closely reflect the lack of functional humoral immunity observed in a battery of sophisticated immune assays, while positive results do not necessarily reflect adequate immunity. After booster vaccination, both groups gain depth and breadth of systemic antibodies against evolving SARS-CoV-2 and related viruses. Our findings show that LFA-based antibody tests can alert individuals about inadequate immunity against COVID-19, thereby increasing booster shots and promoting herd immunity.

IgG and IgM differentiation in a particle-based agglutination assay by control over antigen surface density

Point-of-care (POC) testing offers fast and on-site diagnostics and can be crucial against many infectious diseases and in screening. One remaining challenge in serological POC testing is the quantification of immunoglobulin G (IgG) and immunoglobulin M (IgM). Quantification of IgG/IgM can be important to evaluate immunity and to discriminate recent infections from past infections and primary infections from secondary infections. POC tests such as lateral flow immunoassays allow IgG and IgM differentiation; however, a remaining limitation is their incapacity to provide quantitative results. In this work, we show how samples containing IgG or IgM can be distinguished in a nanoparticle-based agglutination biosensing assay by tuning the density of antigens on the nanoparticles' surface. We employ direct STochastic Optical Reconstruction Microscopy to quantify the accessible SARS-CoV-2 trimeric spike proteins conjugated to magnetic nanoparticles at a single-particle level and gain insight into the protein distribution provided by the conjugation procedure. Furthermore, we measure the anti-SARS-CoV-2 IgG/IgM induced agglutination using an optomagnetic readout principle. We show that particles with high antigen density have a relatively higher sensitivity toward IgM compared to IgG, whereas low antigen density provides a relatively higher sensitivity to IgG. The finding paves the way for its implementation for other agglutination-based serology tests, allowing for more accurate disease diagnosis.

Upcycling of SARS-CoV-2 Rapid Antigen Test Cassettes into Flame Retardant Plastics

The COVID-19 pandemic resulted in the generation of large quantities of medical waste and highlighted the importance of efficient waste management systems. One good example of this is rapid antigen tests, which contain valuable resources, and which are usually incinerated after their use. The present study aimed to evaluate the potential of waste rapid antigen test cassettes (RATCs) as a resource for the preparation of sustainable flame-retardant plastics. Milled RATCs were compounded with different concentrations (10-30 wt.%) of aluminium diethylphosphinate (ADP) and injection moulded into test specimens. Prepared samples were exposed to ultraviolet (UV) ageing for varying durations and characterised by Fourier-transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), tensile tests, Charpy impact tests, and vertical burning tests. FT-IR analysis revealed that RATCs are composed mainly of high-impact polystyrene (HIPS), which was further confirmed by suitable glass transition temperatures (Tg) determined by DSC and DMA. The addition of ADP resulted in progressive embrittlement of HIPS with increasing concentration, while flammability decreased significantly and reached V-1 classification at loading of 30 wt.%. UV ageing caused photo-oxidative degradation of HIPS, which resulted in decreased strain-at-break, while flammability was not affected.

Formation and Detection of Gizzerosine in Animal Feed Matrices: Progress and Perspectives

Gizzerosine is responsible for gizzard erosion and black vomit, owing to excessive gastric acid secretion in poultry. It is a biogenic amine that forms during feed processing. Gizzerosine, a derivative of histamine, is a serious threat to animal feed safety and poultry production because it is more potent after ingestion and more harmful to poultry than histamine. The difficulty of obtaining gizzerosine and the lack of simple, rapid, and sensitive in vitro detection techniques have hindered studies on the effects of gizzerosine on gizzard health and poultry production. In this review, we evaluated the natural formation and the chemical synthesis methods of gizzerosine and introduced seven detection methods and their principles for analyzing gizzerosine. This review summarizes the issues of gizzerosine research and suggests methods for the future development of gizzerosine detection methods.

A semi-quantitative visual lateral flow immunoassay for SARS-CoV-2 antibody detection for the follow-up of immune response to vaccination or recovery

The lateral flow immunoassay (LFIA) technique is largely employed for the point-of-care detection of antibodies especially for revealing the immune response in serum. Visual LFIAs usually provide the qualitative yes/no detection of antibodies, while quantification requires some equipment, making the assay more expensive and complicated. To achieve visual semi-quantification, the alignment of several lines (made of the same antigen) along a LFIA strip has been proposed. The numbering of the reacting lines has been used to correlate with the quantity of some biomarkers in serum. Here, we designed the first semiquantitative LFIA for detecting antibodies and applied it to classify the immune response to SARS-CoV-2 raised by vaccination or natural infection. We used a recombinant spike receptor-binding domain (RBD) as the specific capture reagent to draw two test lines. The detection reagent was selected among three possible ligands that are able to bind to anti-spike human antibodies: the same RBD, staphylococcal protein A, and anti-human immunoglobulin G antibodies. The most convenient detector, adsorbed on gold nanoparticles, was chosen based on the highest correlation with an antibody titre of 171 human sera, measured by a reference serological method, and was the RBD (Spearman's rho = 0.84). Incorporated into the semiquantitative LFIA, it confirmed the ability to discriminate high- and low-titre samples and to classify them into two classes (Dunn's test, P < 0.05). The proposed approach enabled the semiquantification of the immune response to SARS-CoV-2 by the unaided eye observation, thus overcoming the requirement of costly and complicated equipment, and represents a general strategy for the development of semiquantitative serological LFIAs.

Recent Advances in Quantum Dot-Based Lateral Flow Immunoassays for the Rapid, Point-of-Care Diagnosis of COVID-19

The COVID-19 pandemic has spurred demand for efficient and rapid diagnostic tools that can be deployed at point of care to quickly identify infected individuals. Existing detection methods are time consuming and they lack sensitivity. Point-of-care testing (POCT) has emerged as a promising alternative due to its user-friendliness, rapidity, and high specificity and sensitivity. Such tests can be conveniently conducted at the patient's bedside. Immunodiagnostic methods that offer the rapid identification of positive cases are urgently required. Quantum dots (QDs), known for their multimodal properties, have shown potential in terms of combating or inhibiting the COVID-19 virus. When coupled with specific antibodies, QDs enable the highly sensitive detection of viral antigens in patient samples. Conventional lateral flow immunoassays (LFAs) have been widely used for diagnostic testing due to their simplicity, low cost, and portability. However, they often lack the sensitivity required to accurately detect low viral loads. Quantum dot (QD)-based lateral flow immunoassays have emerged as a promising alternative, offering significant advancements in sensitivity and specificity. Moreover, the lateral flow immunoassay (LFIA) method, which fulfils POCT standards, has gained popularity in diagnosing COVID-19. This review focuses on recent advancements in QD-based LFIA for rapid POCT COVID-19 diagnosis. Strategies to enhance sensitivity using QDs are explored, and the underlying principles of LFIA are elucidated. The benefits of using the QD-based LFIA as a POCT method are highlighted, and its published performance in COVID-19 diagnostics is examined. Overall, the integration of quantum dots with LFIA holds immense promise in terms of revolutionizing COVID-19 detection, treatment, and prevention, offering a convenient and effective approach to combat the pandemic.