Commercially available rapid diagnostic tests for the detection of high priority pathogens: status and challenges

A nanoparticle-assisted signal-enhancement technique for lateral flow immunoassays

Lateral flow immunoassay (LFIA), an affordable and rapid paper-based detection technology, is employed extensively in clinical diagnosis, environmental monitoring, and food safety analysis. The COVID-19 pandemic underscored the validity and adoption of LFIA in performing large-scale clinical and public health testing. The unprecedented demand for prompt diagnostic responses and advances in nanotechnology have fueled the rise of next-generation LFIA technologies. The utilization of nanoparticles to amplify signals represents an innovative approach aimed at augmenting LFIA sensitivity. This review probes the nanoparticle-assisted amplification strategies in LFIA applications to secure low detection limits and expedited response rates. Emphasis is placed on comprehending the correlation between the physicochemical properties of nanoparticles and LFIA performance. Lastly, we shed light on the challenges and opportunities in this prolific field.

Gold Nanotapes and Nanopinecones in a Quantitative Lateral Flow Assay for the Cancer Biomarker Carcinoembryonic Antigen

Colorectal cancer is the third most common malignancy and the second leading cause of cancer death globally. Multiple studies have linked levels of carcinoembryonic antigen in patient serum to poor disease prognosis. Hence, the ability to detect low levels of carcinoembryonic antigen has applications in earlier disease diagnosis, assessment, and recurrence monitoring. Existing carcinoembryonic antigen detection methods often require multiple reagents, trained operators, or complex procedures. A method alleviating these issues is the lateral flow assay, a paper-based platform that allows the detection and quantification of target analytes in complex mixtures. The tests are rapid, are point-of-care, possess a long shelf life, and can be stored at ambient conditions, making them ideal for use in a range of settings. Although lateral flow assays typically use spherical gold nanoparticles to generate the classic red signal, recent literature has shown that alternate morphologies to spheres can improve the limit of detection. In this work, we report the application of alternative gold nanoparticle morphologies, gold nanotapes (∼35 nm in length) and gold nanopinecones (∼90 nm in diameter), in a lateral flow assay for carcinoembryonic antigen. In a comparative assay, gold nanopinecones exhibited a ∼2× improvement in the limit of detection compared to commercially available spherical gold nanoparticles for the same antibody loading and total gold content, whereas the number of gold nanopinecones in each test was ∼3.2× less. In the fully optimized test, a limit of detection of 14.4 pg/mL was obtained using the gold nanopinecones, representing a 24-fold improvement over the previously reported gold-nanoparticle-based carcinoembryonic antigen lateral flow assay.

A 3D paper microfluidic device for enzyme-linked assays: Application to DNA analysis

A paper microfluidic device capable of conducting enzyme-linked assays is presented: a microfluidic enzyme-linked paper analytical device (μEL-PAD). The system exploits a wash-free sandwich coupling to form beads/analyte/enzyme complexes, which are subsequently added to the vertical flow device composed of wax-printed paper, waxed nitrocellulose membrane and absorbent/barrier layers. The nitrocellulose retains the bead complexes without disrupting the flow, enabling for an efficient washing step. The entrapped complexes then interact with the chromogenic substrate stored on the detection paper, generating a color change on it, quantified with an open-source smartphone software. This is a universal paper-based technology suitable for high-sensitivity quantification of many analytes, such as proteins or nucleic acids, with different enzyme-linked formats. Here, the potential of the μEL-PAD is demonstrated to detect DNA from Staphylococcus epidermidis. After generation of isothermally amplified genomic DNA from bacteria, Biotin/FITC-labeled products were analyzed with the μEL-PAD, exploiting streptavidin-coated beads and antiFITC-horseradish peroxidase. The μEL-PAD achieved a limit of detection (LOD) and quantification <10 genome copies/μL, these being at least 70- and 1000-fold lower, respectively, than a traditional lateral flow assay (LFA) exploiting immobilized streptavidin and antiFITC-gold nanoparticles. It is envisaged that the device will be a good option for low-cost, simple, quantitative, and sensitive paper-based point-of-care testing.

Reusable, Ultrasensitive, Patterned Conjugated Polyelectrolyte-Surfactant Complex Film with a Wide Detection Range for Copper Ion Detection

Conjugated polyelectrolytes (CPEs) are emerging as promising materials in the sensor field because they enable high-sensitivity detection of various substances in aqueous media. However, most CPE-based sensors have serious problems in real-world application because the sensor system is operated only when the CPE is dissolved in aqueous media. Here, the fabrication and performance of a water-swellable (WS) CPE-based sensor driven in the solid state are demonstrated. The WS CPE films are prepared by immersing a water-soluble CPE film in cationic surfactants of different alkyl chain lengths in a chloroform solution. The prepared film exhibits rapid, limited water swellability despite the absence of chemical crosslinking. The water swellability of the film enables the highly sensitive and selective detection of Cu²⁺ in water. The fluorescence quenching constant and the detection limit of the film are 7.24 × 10⁶ L mol^-1 and 4.38 nM (0.278 ppb), respectively. Moreover, the film is reusable via a facile treatment. Furthermore, various fluorescent patterns introduced by different surfactants are successfully fabricated by a simple stamping method. By integrating the patterns, Cu²⁺ detection in a wide concentration range (nM-mM) can be achieved.

A Comparison Study of the Detection Limit of Omicron SARS-CoV-2 Nucleocapsid by Various Rapid Antigen Tests

Rapid antigen tests (RATs) are widely used worldwide to detect SARS-CoV-2 since they are an easy-to-use kit and offer rapid results. The RAT detects the presence of the nucleocapsid protein, which is located inside the virus. However, the sensitivity of the different RATs varies between commercially available kits. The test result might change due to various factors, such as the variant type, infection date, swab's surface, the manner in which one performs the testing and the mucus components. Here, we compare the detection limit of seven commercially available RATs by introducing them to known SARS-CoV-2 nucleocapsid protein amounts from the Omicron variant. It allows us to determine the detection limit, disregarding the influences of other factors. A lower detection limit of the RAT is necessary since earlier detection will help reduce the spread of the virus and allow faster treatment, which might be crucial for the population at risk.

Portable electroanalytical nucleic acid amplification tests using printed circuit boards and open-source electronics

The realization of electrochemical nucleic acid amplification tests (NAATs) at the point of care (POC) is highly desirable, but it remains a challenge given their high cost and lack of true portability/miniaturization. Here we show that mass-produced, industrial standardized, printed circuit boards (PCBs) can be repurposed to act as near-zero cost electrodes for self-assembled monolayer-based DNA biosensing, and further integration with a custom-designed and low-cost portable potentiostat. To show the analytical capability of this system, we developed a NAAT using isothermal recombinase polymerase amplification, bypassing the need of thermal cyclers, followed by an electrochemical readout relying on a sandwich hybridization assay. We used our sensor and device for analytical detection of the toxic microalgae Ostreopsis cf. ovata as a proof of concept. This work shows the potential of PCBs and open-source electronics to be used as powerful POC DNA biosensors at a low-cost.

Rapid diagnostic tests versus RT-PCR for Ebola virus infections: a systematic review and meta-analysis

Objective

To evaluate the clinical accuracy of rapid diagnostic tests for the detection of Ebola virus.

Methods

We searched MEDLINE®, Embase® and Web of Science for articles published between 1976 and October 2021 reporting on clinical studies assessing the performance of Ebola virus rapid diagnostic tests compared with reverse transcription polymerase chain reaction (RT-PCR). We assessed study quality using the QUADAS-2 criteria. To estimate the pooled sensitivity and specificity of these rapid diagnostic tests, we used a bivariate random-effects meta-analysis.

Findings

Our search identified 113 unique studies, of which nine met the inclusion criteria. The studies were conducted in the Democratic Republic of the Congo, Guinea, Liberia and Sierra Leone and they evaluated 12 rapid diagnostic tests. We included eight studies in the meta-analysis. The pooled sensitivity and specificity of the rapid tests were 86% (95% confidence interval, CI: 80-91) and 95% (95% CI: 91-97), respectively. However, pooled sensitivity decreased to 83% (95% CI: 77-88) after removing outliers. Pooled sensitivity increased to 90% (95% CI: 82-94) when analysis was restricted to studies using the RT-PCR from altona Diagnostics as gold standard. Pooled sensitivity increased to 99% (95% CI: 67-100) when the analysis was restricted to studies using whole or capillary blood specimens.

Conclusion

The included rapid diagnostic tests did not detect all the Ebola virus disease cases. While the sensitivity and specificity of these tests are moderate, they are still valuable tools, especially useful for triage and detecting Ebola virus in remote areas.

Performance Validation of COVID-19 Self-Conduct Buccal and Nasal Swabs RTK-Antigen Diagnostic Kit

The antigen rapid diagnostic test (Ag-RDT) is an immunodiagnostic test that detects the presence of viral proteins (antigens) expressed by the COVID-19 virus in a sample from a patient's respiratory tract. This study focused on evaluating the performance of self-conduct buccal and nasal swabs RTK-antigen test compared to nasopharyngeal swab RTK-based COVID-19 diagnostic assays, Panbio™ COVID-19 Ag Rapid Test Device (Nasopharyngeal) (Abbott Rapid Diagnostics Jena GmbH, Jena, Germany) used in hospitals for first-line screening. The sensitivity and specificity of the paired RTK-Ag test in detecting the an-tigen were calculated at 96.4% and 100%, respectively. Fisher exact tests showed the association between nasopharyngeal swabs RTK-Ag assay and buccal-nasal swabs RTK-Ag from ProdetectTM is significant (p-values < 0.001). The result showed that a self-conducted buccal and nasal RTK-antigen rapid test by the patients is comparable to the results obtained from a rapid test device conducted by trained medical personnel using a nasopharyngeal swab.

Development of an artificial zinc finger - Luciferase fusion protein for the rapid detection of Salmonella typhimurium

A novel artificial Zinc finger - luciferase fusion protein was successfully developed for rapid detection of Salmonella typhimurium, a worldwide-distributed foodborne pathogen. The designed Zinc finger (ZF) protein bound specifically to a 12 bp region of the Salmonella spp invasion gene invA. While the luciferase from Gaussia princeps called Gaussia luciferase (Gluc) was for the first time fused with the artificial ZF domain to improve the detection sensitivity. The fusion protein successfully recognized and bound to the synthesized invA dsDNA with high specificity and sensitivity. The detection limit was as low as 10 fmol of dsNDA. Then, the bacteria PCR products were subsequently used to assess the zinc finger - luciferase fusion protein. The final results indicated that the ZF-Gluc fusion protein system could detect S. typhimurium as low as 1 CFU/mL in 2 h after the PCR. Therefore, this study provided us with a novel artificial zinc finger fusion protein and an efficient method to accomplish the rapid detection of the major foodborne pathogen S. typhimurium. In addition, the specific artificial ZF proteins that bund to particular dsDNA sequences could be easily designed, the ZF-Gluc might has broad application prospects in the field of rapid pathogenic bacteria detection.