Automatic ultrasensitive lateral flow immunoassay based on a color-enhanced signal amplification strategy

Lateral flow immunoassays (LFIAs) are an essential and widely used point-of-care test for medical diagnoses. However, commercial LFIAs still have low sensitivity and specificity. Therefore, we developed an automatic ultrasensitive dual-color enhanced LFIA (DCE-LFIA) by applying an enzyme-induced tyramide signal amplification method to a double-antibody sandwich LFIA for antigen detection. The DCE-LFIA first specifically captured horseradish peroxidase (HRP)-labeled colored microspheres at the Test line, and then deposited a large amount of tyramide-modified signals under the catalytic action of HRP to achieve the color superposition. A limit of detection (LOD) of 3.9 pg/mL and a naked-eye cut-off limit of 7.8 pg/mL were achieved for detecting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleoprotein. Additionally, in the inactivated virus detections, LOD equivalent to chemiluminescence (0.018 TCID50/mL) was obtained, and it had excellent specificity under the interference of other respiratory viruses. High sensitivity has also been achieved for detection of influenza A, influenza B, cardiac troponin I, and human chorionic gonadotrophin using this DCE-LFIA, suggesting the assay is universally applicable. To ensure the convenience and stability in practical applications, we created an automatic device. It provides a new practical option for point-of-care test immunoassays, especially ultra trace detection and at-home testing.

A high throughput immuno-affinity mass spectrometry method for detection and quantitation of SARS-CoV-2 nucleoprotein in human saliva and its comparison with RT-PCR, RT-LAMP, and lateral flow rapid antigen test

OBJECTIVES

Many reverse transcription polymerase chain reaction (RT-PCR) methods exist that can detect SARS-CoV-2 RNA in different matrices. RT-PCR is highly sensitive, although viral RNA may be detected long after active infection has taken place. SARS-CoV-2 proteins have shorter detection windows hence their detection might be more meaningful. Given salivary droplets represent a main source of transmission, we explored the detection of viral RNA and protein using four different detection platforms including SISCAPA peptide immunoaffinity liquid chromatography-mass spectrometry (SISCAPA-LC-MS) using polyclonal capture antibodies.

METHODS

The SISCAPA-LC MS method was compared to RT-PCR, RT-loop-mediated isothermal amplification (RT-LAMP), and a lateral flow rapid antigen test (RAT) for the detection of virus material in the drool saliva of 102 patients hospitalised after infection with SARS-CoV-2. Cycle thresholds (Ct) of RT-PCR (E gene) were compared to RT-LAMP time-to-positive (TTP) (NE and Orf1a genes), RAT optical densitometry measurements (test line/control line ratio) and to SISCAPA-LC-MS for measurements of viral protein.

RESULTS

SISCAPA-LC-MS showed low sensitivity (37.7 %) but high specificity (89.8 %). RAT showed lower sensitivity (24.5 %) and high specificity (100 %). RT-LAMP had high sensitivity (83.0 %) and specificity (100.0 %). At high initial viral RNA loads (<20 Ct), results obtained using SISCAPA-LC-MS correlated with RT-PCR (R2 0.57, p-value 0.002).

CONCLUSIONS

Detection of SARS-CoV-2 nucleoprotein in saliva was less frequent than the detection of viral RNA. The SISCAPA-LC-MS method allowed processing of multiple samples in <150 min and was scalable, enabling high throughput.

Highly Stable Buffer-Based Zinc Oxide/Reduced Graphene Oxide Nanosurface Chemistry for Rapid Immunosensing of SARS-CoV-2 Antigens

The widespread and long-lasting effect of the COVID-19 pandemic has called attention to the significance of technological advances in the rapid diagnosis of SARS-CoV-2 virus. This study reports the use of a highly stable buffer-based zinc oxide/reduced graphene oxide (bbZnO/rGO) nanocomposite coated on carbon screen-printed electrodes for electrochemical immuno-biosensing of SARS-CoV-2 nuelocapsid (N-) protein antigens in spiked and clinical samples. The incorporation of a salt-based (ionic) matrix for uniform dispersion of the nanomixture eliminates multistep nanomaterial synthesis on the surface of the electrode and enables a stable single-step sensor nanocoating. The immuno-biosensor provides a limit of detection of 21 fg/mL over a linear range of 1-10 000 pg/mL and exhibits a sensitivity of 32.07 ohms·mL/pg·mm² for detection of N-protein in spiked samples. The N-protein biosensor is successful in discriminating positive and negative clinical samples within 15 min, demonstrating its proof of concept used as a COVID-19 rapid antigen test.

SARS-CoV-2 RNA and antibody detection in breast milk from a prospective multicentre study in Spain

OBJECTIVES

To develop and validate a specific protocol for SARS-CoV-2 detection in breast milk matrix and to determine the impact of maternal SARS-CoV-2 infection on the presence, concentration and persistence of specific SARS-CoV-2 antibodies.

DESIGN AND PATIENTS

This is a prospective, multicentre longitudinal study (April-December 2020) in 60 mothers with SARS-CoV-2 infection and/or who have recovered from COVID-19. A control group of 13 women before the pandemic were also included.

SETTING

Seven health centres from different provinces in Spain.

MAIN OUTCOME MEASURES

Presence of SARS-CoV-2 RNA in breast milk, targeting the N1 region of the nucleocapsid gene and the envelope (E) gene; presence and levels of SARS-CoV-2-specific immunoglobulins (Igs)-IgA, IgG and IgM-in breast milk samples from patients with COVID-19.

RESULTS

All breast milk samples showed negative results for presence of SARS-CoV-2 RNA. We observed high intraindividual and interindividual variability in the antibody response to the receptor-binding domain of the SARS-CoV-2 spike protein for each of the three isotypes IgA, IgM and IgG. Main Protease (MPro) domain antibodies were also detected in milk. 82.9% (58 of 70) of milk samples were positive for at least one of the three antibody isotypes, with 52.9% of these positive for all three Igs. Positivity rate for IgA was relatively stable over time (65.2%-87.5%), whereas it raised continuously for IgG (from 47.8% for the first 10 days to 87.5% from day 41 up to day 206 post-PCR confirmation).

CONCLUSIONS

Our study confirms the safety of breast feeding and highlights the relevance of virus-specific SARS-CoV-2 antibody transfer. This study provides crucial data to support official breastfeeding recommendations based on scientific evidence. Trial registration number NCT04768244.

Estimating SARS-CoV-2 Seroprevalence in Canadian Blood Donors, April 2020 to March 2021: Improving Accuracy with Multiple Assays

We have previously used composite reference standards and latent class analysis (LCA) to evaluate the performance of laboratory assays in the presence of tarnished gold standards. Here, we apply these techniques to repeated, cross-sectional study of Canadian blood donors, whose sera underwent parallel testing with four separate SARS-CoV-2 antibody assays. We designed a repeated cross-sectional design with random cross-sectional sampling of all available retention samples (n = 1500/month) for a 12 -month period from April 2020 until March 2021. Each sample was evaluated for SARS-CoV-2 IgG antibodies using four assays an Abbott Architect assay targeting the nucleocapsid antigen (Abbott-NP, Abbott, Chicago IL) and three in-house IgG ELISAs recognizing distinct recombinant viral antigens: full-length spike glycoprotein (Spike), spike glycoprotein receptor binding domain (RBD) and nucleocapsid (NP). We used two analytic approaches to estimate SAR-CoV-2 seroprevalence: a composite reference standard and LCA. Using LCA to estimate true seropositivity status based on the results of the four antibody tests, we estimated that seroprevalence increased from 0.8% (95% CI: 0.5-1.4%) in April 2020 to 6.3% (95% CI: 5.1-7.6%) in March 2021. Our study provides further support for the use of LCA in upcoming public health crises, epidemics, and pandemics when a gold standard assay may not be available or identifiable. IMPORTANCE Here, we describe an approach to estimating seroprevalence in a low prevalence setting when multiple assays are available and yet no known gold standard exists. Because serological studies identify cases through both diagnostic testing and surveillance, and otherwise silent, unrecognized infections, serological data can be used to estimate the true infection fatality ratio of a disease. However, seroprevalence studies rely on assays with imperfect sensitivity and specificity. Seroreversion (loss of antibody response) also occurs over time, and with the advent of vaccination, distinction of antibody response resulting from vaccination as opposed to antibody response due to infection has posed an additional challenge. Our approach indicates that seroprevalence on Canadian blood donors by the end of March 2021was less than 10%. Our study supports the use of latent class analysis in upcoming public health crises, epidemics, and pandemics when a gold standard assay may not be available or identifiable.

Immunohistochemistry for protein detection in PFA-fixed paraffin-embedded SARS-CoV-2-infected COPD airway epithelium

Patients with chronic lung disease are vulnerable to getting severe diseases associated with SARS-CoV-2 infection. Here, we describe protocols for subculturing and differentiating primary normal human bronchial epithelial (NHBE) cells of patients with chronic obstructive lung disease. The differentiation of NHBE cells in air-liquid interface mimics an in vivo airway and provides an in vitro model for studying SARS-CoV-2 infection. We also describe a protocol for detecting proteins in the sectioned epithelium for detailing SARS-CoV-2 infection-induced pathobiology with a vertical view.

Ultrasensitive and Simultaneous Detection of Two Specific SARS-CoV-2 Antigens in Human Specimens Using Direct/Enrichment Dual-Mode Fluorescence Lateral Flow Immunoassay

Sensitive point-of-care methods for detecting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antigens in clinical specimens are urgently needed to achieve rapid screening of viral infection. We developed a magnetic quantum dot-based dual-mode lateral flow immunoassay (LFIA) biosensor for the high-sensitivity simultaneous detection of SARS-CoV-2 spike (S) and nucleocapsid protein (NP) antigens, which is beneficial for improving the detection accuracy and efficiency of SARS-CoV-2 infection in the point-of-care testing area. A high-performance magnetic quantum dot with a triple-QD shell (MagTQD) nanotag was first fabricated and integrated into the LFIA system to provide superior fluorescence signals, enrichment ability, and detectability for S/NP antigen testing. Two detection modes were provided by the proposed MagTQD-LFIA. The direct mode was used for rapid screening or urgent detection of suspected samples within 10 min, and the enrichment mode was used for the highly sensitive and quantitative analysis of SARS-CoV-2 antigens in biological samples without the interference of the "hook effect." The simultaneous detection of SARS-CoV-2 S/NP antigens was conducted in one LFIA strip, and the detection limits for two antigens under direct and enrichment modes were 1 and 0.5 pg/mL, respectively. The MagTQD-LFIA showed high accuracy, specificity, and stability in saliva and nasal swab samples and is an efficient tool with flexibility to meet the testing requirements for SARS-CoV-2 antigens in various situations.

A Generic, Scalable, and Rapid Time-Resolved Förster Resonance Energy Transfer-Based Assay for Antigen Detection-SARS-CoV-2 as a Proof of Concept

The ongoing coronavirus disease 2019 (COVID-19) pandemic has seen an unprecedented increase in the demand for rapid and reliable diagnostic tools, leaving many laboratories scrambling for resources. We present a fast and simple assay principle for antigen detection and demonstrate its functionality by detecting severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antigens in nasopharyngeal swabs. The method is based on the detection of SARS-CoV-2 nucleoprotein (NP) and S protein (SP) via time-resolved Förster resonance energy transfer (TR-FRET) with donor- and acceptor-labeled polyclonal anti-NP and -SP antibodies. Using recombinant proteins and cell culture-grown SARS-CoV-2, the limits of detection were established as 25 pg of NP or 20 infectious units (IU) and 875 pg of SP or 625 IU. Testing reverse transcription-PCR (RT-PCR)-positive (n = 48, with cycle threshold [CT ] values from 11 to 30) or -negative (n = 96) nasopharyngeal swabs demonstrated that the assay yielded positive results for all samples with CT values of <25 and for a single RT-PCR-negative sample. Virus isolation from the RT-PCR-positive nasopharyngeal swabs showed a strong association between the presence of infectious virus and a positive antigen test result. The NP-based assay showed 97.4% (37/38) sensitivity and 100% (10/10) specificity in comparison with virus isolation and 77.1% (37/48) sensitivity and 99.0% (95/96) specificity in comparison with SARS-CoV-2 RT-PCR. The assay is performed in a buffer that neutralizes SARS-CoV-2 infectivity, and the assay is relatively simple to set up as an "in-house" test. Here, SARS-CoV-2 served as the model pathogen, but the assay principle is applicable to other viral infections, and the test format could easily be adapted to high-throughput testing.IMPORTANCE PCR is currently the gold standard for the diagnosis of many acute infections. While PCR and its variants are highly sensitive and specific, the time from sampling to results is measured in hours at best. Antigen tests directly detect parts of the infectious agent, which may enable faster diagnosis but often at lower sensitivity and specificity. Here, we describe a technique for rapid antigen detection and demonstrate the test format's potential using SARS-CoV-2 as the model pathogen. The 10-min test, performed in a buffer that readily inactivates SARS-CoV-2, from nasopharyngeal samples identified 97.4% (37/38) of the samples from which we could isolate the virus. This suggests that the test performs well in identifying patients potentially shedding the virus. Although SARS-CoV-2 served as the model pathogen to demonstrate proof of concept, the test principle itself would be applicable to a wide variety of infectious and perhaps also noninfectious diseases.

Protein-based lateral flow assays for COVID-19 detection

To combat the enduring and dangerous spread of COVID-19, many innovations to rapid diagnostics have been developed based on proteinprotein interactions of the SARS-CoV-2 spike and nucleocapsid proteins to increase testing accessibility. These antigen tests have most prominently been developed using the lateral flow assay (LFA) test platform which has the benefit of administration at point-of-care, delivering quick results, lower cost, and does not require skilled personnel. However, they have gained criticism for an inferior sensitivity. In the last year, much attention has been given to creating a rapid LFA test for detection of COVID-19 antigens that can address its high limit of detection while retaining the advantages of rapid antibodyantigen interaction. In this review, a summary of these proteinprotein interactions as well as the challenges, benefits, and recent improvements to protein based LFA for detection of COVID-19 are discussed.

Development and performance evaluation of a rapid in-house ELISA for retrospective serosurveillance of SARS-CoV-2

BACKGROUND

In the ongoing pandemic situation of COVID-19, serological tests can complement the molecular diagnostic methods, and can be one of the important tools of sero-surveillance and vaccine evaluation.

AIM

To develop and evaluate a rapid SARS-CoV-2 specific ELISA for detection of anti-SARS-CoV2 IgG from patients' biological samples.

METHODS

In order to develop this ELISA, three panels of samples (n = 184) have been used: panel 1 (n = 19) and panel 2 (n = 60) were collected from RT-PCR positive patients within 14 and after 14 days of onset of clinical symptoms, respectively; whereas panel 3 consisted of negative samples (n = 105) collected either from healthy donors or pre-pandemic dengue patients. As a capturing agent full-length SARS-CoV2 specific recombinant nucleocapsid was immobilized. Commercial SARS-CoV2 IgG kit based on chemiluminescent assay was used for the selection of samples and optimization of the assay. The threshold cut-off point, inter-assay and intra-assay variations were determined.

RESULTS

The incubation/reaction time was set at a total of 30 minutes with the sensitivity of 84% (95% confidence interval, CI, 60.4%, 96.6%) and 98% (95% CI, 91.1%, 100.0%), for panel 1 and 2, respectively; with overall 94.9% sensitivity (95% CI 87.5%, 98.6%). Moreover, the clinical specificity was 97.1% (95% CI, 91.9%, 99.4%) with no cross reaction with dengue samples. The overall positive and negative predictive values are 96.2% (95% CI 89.2%, 99.2%) and 96.2% (95% CI, 90.6% 99.0%), respectively. In-house ELISA demonstrated 100% positive and negative percent agreement with Elecsys Anti-SARS-CoV-2, with Cohen's kappa value of 1.00 (very strong agreement), while comparing 13 positive and 17 negative confirmed cases.

CONCLUSION

The assay is rapid and can be applied as one of the early and retrospective sero-monitoring tools in all over the affected areas.

Clinical Evaluation of BD Veritor SARS-CoV-2 Point-of-Care Test Performance Compared to PCR-Based Testing and versus the Sofia 2 SARS Antigen Point-of-Care Test

The clinical performance of the BD Veritor System for Rapid Detection of SARS-CoV-2 nucleocapsid antigen (Veritor), a chromatographic immunoassay used for SARS-CoV-2 point-of-care testing, was evaluated using nasal specimens from individuals with COVID-19 symptoms. Two studies were completed to determine clinical performance. In the first study, nasal specimens and either nasopharyngeal or oropharyngeal specimens from 251 participants with COVID-19 symptoms (≤7 days from symptom onset [DSO], ≥18 years of age) were utilized to compare Veritor with the Lyra SARS-CoV-2 PCR assay (Lyra). In the second study, nasal specimens from 361 participants with COVID-19 symptoms (≤5 DSO, ≥18 years of age) were utilized to compare performance of Veritor to that of the Sofia 2 SARS Antigen FIA test (Sofia 2). The positive, negative, and overall percent agreement (PPA, NPA, and OPA, respectively) were the primary outcomes. In study 1, the PPA for Veritor, compared to Lyra, ranged from 81.8 to 87.5% across the 0 to 1 and 0 to 6 DSO ranges. In study 2, Veritor had PPA, NPA, and OPA values of 97.4, 98.1, and 98.1%, respectively, with Sofia 2. Discordant analysis showed one Lyra positive missed by Veritor and five Lyra positives missed by Sofia 2; one Veritor positive result was negative by Lyra. Veritor met FDA emergency use authorization (EUA) acceptance criteria for SARS-CoV-2 antigen testing for the 0 to 5 and 0 to 6 DSO ranges (PPA values of 83.9% and 82.4%, respectively). Veritor and Sofia 2 showed a high degree of agreement for SARS-CoV-2 detection. The Veritor test allows for more rapid COVID-19 testing utilizing easy-to-collect nasal swabs but demonstrated <100% PPA compared to PCR.