An Enzyme Linked Immunosorbent Assay (ELISA) is an immunological assay (immunoassay) used to measure antibodies, antigens, proteins and glycoproteins in biological samples. ELISA assays are used as diagnostic tests and for measurement of cytokines or soluble receptors in a cell supernatant or serum. ELISAs use chromogenic reactions to detect captured antigen-antibody complexes. ELISA assays are used for rapid HIV detection using blood or saliva as well as detection of many viruses, bacteria, fungi, autoimmune diseases, food allergens, blood typing, pregnancy hormone hCG. ELISAs are also used for forensic toxicology and research.
Antigenic proteins or antibodies are bound to a solid support or 96 well microtiter plate. Fluid, serum or clinical sample is added to the plate. An antigen on the plate will capture antibodies in the sample. An antibody on the plate will capture a specific protein or antigen. To detect the presence of bound antibody or antigen ELISAs use enzymes such as horse radish peroxidase or alkaline phosphatase for chromogenic reactions where a substrate is converted into a colored product that can be measured using a plate reader. These enzymes are conjugated to detection antibodies that bind to the antibodies or proteins that were captured on the plate.
ELISA’s are in development for diagnosis of present or past SARS-CoV-2 infection which causes COVID-19 coronavirus disease that causes respiratory illness. Serology assays such as ELISA typically seek to detect neutralising antibodies produced in the patient against the viral spike (S) glycoprotein. To do this, laboratories produce versions of the spike protein and receptor binding domain (RBD) that can be stably produced in the lab and that can be used in ELISA assays to bind to antibodies present in patient serum.
Testing for the presence of antibodies to the SARS-CoV-2 coronavirus in patient samples done using ELISA and lateral flow immunoassay (LFIA) devices was evaluated by the National COVID Scientific Advisory Panel in May 2020. The ELISA and LFIA assays tested for IgM and IgG antibodies in a laboratory at the University of Oxford using plasma samples from individuals that had confirmed COVID infection based on PCR and from banked pre-pandemic negative control samples. LFIA devices were not found to perform sufficiently well for individual patient applications. The researchers concluded that ELISA can be calibrated to be specific for detecting and quantifying SARS-CoV-2 IgM and IgG and was sensitive for IgG from 10 days after the onset of symptoms. Their ELISA assay recognized antibodies to the spike protein using a recombinant SARS-CoV-2 trimeric spike protein. For their ELISAs, SARS-CoV-2 trimeric S protein was bound to immunoassay plates. When plasma was added to the plates, antibodies that recognized the S protein were captured. The IgG antibodies to the SARS-CoV-2 S protein were detected by a secondary antibody that detects IgG and is conjugated to alkaline phosphatase, an enzyme that produces a chromogenic reaction.