There are two main types of test kits being used to fight the Covid-19 pandemic — antigen tests, which detect if a patient is currently infected — and antibody tests, which reveal if the subject has previously had the virus.
Antigen tests, also referred to as nucleic acid tests, involve swab collection of a genetic sample from inside a person’s nose or mouth. These tests rely on real-time reverse transcription polymerase chain reaction, or RT-PCR, and are most sensitive in the early phases of infection. An ideal diagnostic tool is both specific and sensitive, which means that people who test positive truly have the disease and none of the people carrying the virus slip through the test as a false negative. RT-PCR meets both criteria, with specificity and sensitivity rates of 90 percent and above. The trade-off for RT-PCR’s accuracy is speed; even the fastest tests take at least a few hours, and require trained professionals and large bulky, costly equipment.
Antibody tests, also known as serological tests or immunoassays, make up most of the rapid test kits currently on the market. Antibody tests target the immune response of the person, looking out for antibodies that have developed in response to the virus already circulating in the body for several days. The two types of antibodies detected in response to COVID-19 include Immunoglobulin G (IgG), which is the most common antibody the develops to protect against infections; and Immunoglobulin M (IgM), which is the first antibody the body makes when it fights a new infection. Antibody tests are fast, portable and can be used by non-healthcare professionals, all the things RT-PCR tests are not. The problem is that antibodies only develop several weeks after an infection, which means that antibody-based tests might miss asymptomatic cases or people in the earliest stage of the disease. Additionally, the accuracy of rapid test kits have been recently questioned, with many governments returning shipments of the kits after finding that they had an accuracy of less than 30 percent.
So far, all of the COVID-19 diagnostics that have received the FDA’s Emergency Use Authorization have been antigen test kits for hospital use. But many immunoassays are in the pipeline. What does not exist is an at-home, sensitive, disposable test for COVID-19, similar to a pregnancy test. Such home-based test kits which are cheap, fast, portable and reliable may be a game changer and break lengthy isolation periods and lockdown laws.