Diagnostics for SARS-CoV-2 infections

Abstract

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has spread to nearly every corner of the globe, causing societal instability. The resultant coronavirus disease 2019 (COVID-19) leads to fever, sore throat, cough, chest and muscle pain, dyspnoea, confusion, anosmia, ageusia and headache. These can progress to life-threatening respiratory insufficiency, also affecting the heart, kidney, liver and nervous systems. The diagnosis of SARS-CoV-2 infection is often confused with that of influenza and seasonal upper respiratory tract viral infections. Due to available treatment strategies and required containments, rapid diagnosis is mandated. This Review brings clarity to the rapidly growing body of available and in-development diagnostic tests, including nanomaterial-based tools. It serves as a resource guide for scientists, physicians, students and the public at large.

Figure 1.. SARS-CoV-2 serological testing.

Commonly used immune-based tests contain SARS-CoV-2 specific recombinant antigens immobilized onto nitrocellulose membranes. Mouse anti-human IgM and IgG antibodies conjugated with colored latex beads are immobilized on conjugate pads. The test sample contacts the membrane within the test. The colored antibodies form latex conjugate complexes with human antiviral antibodies. This complex immobilized on the membrane is captured by the SARS-CoV-2 specific recombinant antigen. If SARS-CoV-2 virus-specific IgG/IgM are present in the sample, it leads to a colored band indicating a positive test result. The complex is captured on the membrane by goat anti-mouse antibody forming a red control line. A built-in control line appears in the test window. The absence of a colored band demonstrates a negative result. The workflow begins with patient serum added to the sample flow well. (1) Saline buffer is added dropwise (2) onto the serum sample (3) until the rabbit antibody-gold shows in the control (C) well. A positive test band indicates the presence of COVID-19 antibody (4–6). Results without a positive C band are invalid (7). Notably, this assay depicts a post-immune response and may show negative results for recently infected patients. It may also detect virus in previously infected but asymptomatic persons.

Figure 2.. RT-PCR and LAMP assays for detection of SARS-CoV-2 infection.

a, A nasopharyngeal swab collects patient samples. 1. RNA is extracted from fluids that contain SARS-CoV-2-infected cells and free viral particles, 2–3. The recovered viral RNA is then reverse transcribed to cDNA and amplified for detection of viral nucleic acids. 4. Conserved regions of the RdRP and E genes are the sub-genomic viral segments amplified with a fluorogenic probe by quantitative PCR. 5. Positive cases exceed the threshold of detection. b, Description of the SARS-CoV-2 RT-loop-mediated isothermal amplification (LAMP) assay. (i), amplification mixtures; (ii), RT-LAMP reaction, and (iii), the products of SARS-CoV-2 RT-LAMP reactions. Although RT-PCR methods are used as the standard for detection of SARS-CoV-2 due to high sensitivity, limitations are present. As an alternative, isothermal amplification methods or LAMP was developed. When optimized for detection, the assay is as sensitive as standard PCR detecting < 10 viral copies/reaction. c, SARS-CoV-2 saliva test. The illustration demonstrates SARS-CoV-2 infection in salivary glands and released specific biomarkers that accumulate in the oral cavity. These are collected through a sample tube and tagged with specific biomarker protein and run through lateral flow rapid tests.

Figure 3.. Nanomaterial-based SARS-COV-2 detections.

Represented illustration of nanomaterial-based biosensors for SARS-CoV-2 detection. Top panel: A field effect transistor (FET)-based biosensing device for detecting SARS-CoV-2. The sensor was produced by FET coated graphene sheets with specific antibodies against the SARS-CoV-2 spike protein. Second panel: The dual functional plasmonic photothermal (PPT) and localized surface plasmon resonance on two-dimensional gold nanoislands functionalized with complementary DNA receptors for detection of the selected SARS-CoV-2 sequences by nucleic acid hybridization. This assays whether a sample contains SARS-CoV-2 RNA. Bottom panel: Nucleic acids or antibodies functionalized materials for SARS-CoV-2 detection by colorimetric and antigen-binding assays. Abbreviations and acronyms: antibody (Ab), antigen (Ag), Förster resonance energy transfer (FRET), nanoparticles (NPs). Schematic ideas and technical methological details were followed and as represented in previously published reports^{, ,} .

Figure 4.. CT and MRI examination of the lung and brain in life threatening SARS-CoV-2 infections.

a, Comparison is shown between lung from a uninfected (left) and a SARS-CoV-2-infected person’s lung tissue (right) in a representative CT scan. In these images, ground-glass opacification (light hazy grey tissue) are seen due to inflammatory responses. ARDS results in fluid accumulation in affected lung tissue independent of cardiac dysfunction (noncardiogenic pulmonary edema). Viral infection causes lung injury leading to loss of gas exchange, atelectasis and hypoxemia. ARDS is associated with fibrinous organizing pneumonia and alveolar damage. SARS-CoV-2 causes epithelial infection and alveolar macrophage inflammation, activation and secretion of a range of pro-inflammatory and chemotactic factors that lead to progressive lung tissue damage. b, SARS-CoV-2-infected patient’s brain MRI scan image showing the brain regions typically involved in those who develop encephalitis or acute necrotizing encephalopathy. The rims of the lateral ventricles can illustrate contrast enhancement with meningeal involvements (red arrow). The medial temporal lobes (yellow arrows), including the hippocampi, may show hyperintense signals, indicating inflammation that may result from the “cytokine storm syndrome”, and hypointense signals that show hemorrhage. Other brain regions including the thalamus, cerebral white matter, brain stem and the cerebellum can be involved.