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Anti-SARS-CoV‑2 IgG --- Rapid and Sensitive Detection

A quick and precise laboratory method for the diagnosis of COVID-19 is desired, but there are major challenges with the huge number of samples and complex situations in hospitals. The key challenges facing hospital laboratories, especially those in rural areas where resources are insufficient, are the setting of standards for specimen collection and nucleic acid extraction as well as the proper testing environment and the correct interpretation of test results. By reverse transcription (RT-)PCR, several variables could lead to false negative results, and the positive rate of the first RT-PCR test for SARS-CoV-2 was only 70 percent.

Authors from Experimental Center of Teaching and Scientific Research, School of Laboratory Medicine and Biotechnology, Southern Medical University, Guangzhou, China report the development of a rapid and responsive immunoassay of lateral flow (LFIA) that detects anti-SARV-CoV-2 IgG in human serum using lanthanide-doped polysterene nanoparticles (LNPs). To capture specific IgG, a recombinant nucleocapsid phosphoprotein of SARS-CoV-2 was dispensed onto a nitrocellulose membrane. 

For this assay, a 100-μL aliquot (1:1000 dilution) of serum samples was used and the entire detection process took 10 minutes. For clinical diagnostic reagents, the findings of the validation experiment fulfilled the requirements.

This study was publish in Analytical Chemistry under title " Rapid and Sensitive Detection of anti-SARS-CoV‑2 IgG, Using Lanthanide-Doped Nanoparticles-Based Lateral Flow Immunoassay".

Mini emulsion polymerisation was used to synthesised the LNPs. 200 mg of Eu-(DPP)3Phen and 20 mg of AIBN were dissolved in 2 g of St in a glass vial, and 60 mg of CTMA and 100 mg of AA were dissolved in 15 mL of deionized water, followed by mouse anti-human IgG antibody (MHIgG) and rabbit IgG antibody were used for the functionalization of LNPs (RIgG).

A 0.0666 value was specified by assaying 51 normal samples as the cutoff value. We screened 7 reverse transcription (RT-)PCR positive samples and 12 negative but clinically suspect samples for anti-SARS-CoV-2 IgG were also present.

For samples with high clinical suspicion that were found to be negative for SARS-CoV-2 by RT-PCR, chest computed tomography (CT) is being used as a confirmatory diagnostic tool. The benefits of being simple and fast are immunodiagnostic techniques such as this lateral flow immunoassay (LFIA).

In aqueous conditions, due to energy loss through the non-radiative activation of the 5D0 state to the O−H vibration of water molecules, the emission of lanthanide chelates is strongly suppressed. The encapsulation of lanthanide chelates in water-dispersible spheres is therefore preferable.

At ~615 nm, the fluorescence emission peak of LNPs was achieved; the peak was small and symmetrical. For physical characterization of LNPs before and after functionalization, dynamic light scattering analysis was performed. The improvement in LNPs' size and surface zeta potential suggested that the antibodies M-HIgG and RIgG were successfully conjugated.

the author further concluded "Based on intra-assay and inter-assay variations, the reproducibility of our assay was  also assessed". For five consecutive days, four samples were tested five times a day and the coefficient of variation (CV) values were determined. 7.71 percent-9.69 percent and 11.51 percent-14.63 percent respectively, were the intra-assay and inter-assay CVs. Since all the CVs were <15%, thus thei assay was reproducible.

A simple and rapid immunoassay based on LNP for the detection of anti-SARS-CoV-2 IgG in human serum is defined in the present study. As no official anti-SARS-CoV-2 IgG standard is currently available, the assay cannot be enhanced from semiquantitative to precise quantification. However for clinical diagnostic reagents, the findings of the validation experiment satisfy the requirements. Our test can be used to monitor the development of COVID-19, as well as therapy responses. By enabling timely diagnosis through early detection of SARS-CoV-2, we expect this assay to be highly useful to help contain the COVID-19 outbreak.

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