What are the different types of serological tests?

There are three types of serological tests that differ in terms of difficulty, sensitivity, and cost:

Rapid Diagnostic Test (RDT): duration 10-30 min.

It is generally a qualitative (positive or negative) side-flow test, small, and portable. These tests can use blood samples from a finger prick, saliva samples, or nasal swabs. TDRs are often similar to pregnancy tests in that the test shows the user coloured lines to indicate positive or negative results. For COVID-19, these tests most often look for the patient's antibodies (IgG and IgM) or for the viral antigen. These tests cannot quantify the amount of antibodies present in the patient's serum nor can they determine whether these antibodies are capable of preventing future infection.

Enzyme-linked immunosorbent assay (ELISA): duration 1 - 5 hours

This test can be qualitative or quantitative and is usually performed in a laboratory. These tests generally use samples of whole blood, or blood fractions from patients. The test is based on the use of a plate coated with a viral protein of interest, such as the "Spike" protein of the viral envelope. Patient samples are then incubated with the protein, and if the patient has antibodies to the viral protein, they bind together. The antibody-bound protein complex can then be detected by another 'antibody' that produces a coloured or fluorescent signal. In COVID-19, these tests most often detect the presence of antibodies produced by the patient (IgG and IgM). However, these tests cannot determine whether these antibodies are neutralizing, i.e., able to protect against future infection of the same virus.

Neutralization test: duration 3- 5 days

This test detects the presence of antibodies that can prevent viral infection in cells in the laboratory. Neutralization tests can tell researchers whether a patient has active and effective antibodies to the virus, even if they are asymptomatic or cured. These tests require samples of the patient's blood. Neutralization tests are based on cell culture, a laboratory method to allow the SARS-CoV-2 virus to multiply in cells. When the virus and cells are cultured with decreasing concentrations of the patient's antibodies, researchers can visualize and determine the dose of antibodies in the patient's blood that can block the virus from multiplying. These tests can determine whether the antibodies are capable of protecting against a future infection but cannot detect the presence of antibodies to viral proteins that are not involved in the infection process.