Recent advances in point of care testing for COVID-19 detection

Abstract

The World Health Organizations declaration of the COVID-19 pandemic was a milestone for the scientific community. The high transmission rate and the huge number of deaths, along with the lack of knowledge about the virus and the evolution of the disease, stimulated a relentless search for diagnostic tests, treatments, and vaccines. The main challenges were the differential diagnosis of COVID-19 and the development of specific, rapid, and sensitive tests that could reach all people. RT-PCR remains the gold standard for diagnosing COVID-19. However, new methods, such as other molecular techniques and immunoassays emerged. Also, the need for accessible tests with quick results boosted the development of point of care tests (POCT) that are fast, and automated, with high precision and accuracy. This assay reduces the dependence on laboratory conditions and mass testing of the population, dispersing the pressure regarding screening and detection. This review summarizes the advances in the diagnostic field since the pandemic started, emphasizing various laboratory techniques for detecting COVID-19. We reviewed the main existing diagnostic methods, as well as POCT under development, starting with RT-PCR detection, but also exploring other nucleic acid techniques, such as digital PCR, loop-mediated isothermal amplification-based assay (RT-LAMP), clustered regularly interspaced short palindromic repeats (CRISPR), and next-generation sequencing (NGS), and immunoassay tests, and nanoparticle-based biosensors, developed as portable instruments for the rapid standard diagnosis of COVID-19.

Keywords: COVID-19; CRISPR; Diagnosis; Immunoassay; Nanobiosensors; Point-of-care testing; RT-LAMP; RT-PCR; SARS-CoV-2.

Graphical abstract

Fig. 1

Schematic representation of SARS-CoV-2 virus structure. The virus has four structural proteins, S (spike), E (envelope), M (membrane), and N (nucleocapsid) proteins. The N protein holds the RNA genome while the S, E, and M proteins together create the viral envelope.

Fig. 2

SARS-CoV-2 virology (simplified, not to scale) and targets for diagnostic methods. SARS-CoV-2 binds to ACE2 receptor in human target cells and subsequently is internalized by endocytosis. Finally, the viral RNA is released for replication and translation by the host cell machinery and further assembly and exocytosis of new viral particles. The main diagnostic targets will depend on virology phase and they could be: molecular techniques, antibody or antigen detection and chest-ct and clinical features.

Fig. 3

Lateral flow immunoassay for COVID-19 detection (LFA). A simple LFA test strip consists of a sample pad, a conjugate pad, test line, control line, and an absorption pad. The sample containing the target analyte is absorbed by the sample pad and it moves toward the conjugate pad. The analyte can interacts with a specific antibody or antigen (labeled with a colored molecule) and forms a mobile conjugate which flows onto the nitrocellulose membrane. The conjugates which are complementary to the immobilized bioreceptors on the test and the control lines get captured. As a result, a change in the color of the lines can then be seen. The test is only valid if the control line is completely visible.

Fig. 4

Schematic diagram of different components of nanobiosensors.