There are two main types of COVID-19 tests: 

1) Diagnostic tests. Types include:
- Molecular tests
- Antigen tests 
2) Antibody (serology) tests

Diagnostic tests can tell you if you have an active COVID-19 infection. Antibody tests can tell you if you’ve been infected by the virus that causes COVID-19 in the past. 

Types of diagnostic tests include molecular tests and antigen tests. Diagnostic tests -- both molecular and antigen tests -- work by looking for proof that the virus is making copies of itself inside the body. A virus making copies inside a “host” (in this case a body) is what makes an active infection, which diagnostic tests can identify. Molecular and antigen tests identify the presence of this copying virus in different ways.

Molecular tests:

Molecular tests look for viral genomic material specific to the COVID-19 virus SARS-CoV-2 (that is, the genetic material and sequencing specific to the COVID-19 virus). Types of molecular tests include nucleic acid amplification tests (NAAT), polymerase chain reaction (PCR) tests, loop-mediated isothermal amplification (LAMP) tests, and clustered, regularly interspaced short palindromic repeat (CRISPR)-based assays tests. 

Molecular tests are usually performed using a nasal swab (most molecular tests) or with a saliva sample (some molecular tests). Nasal swabs for molecular tests can be either shallow or deep in the nose (nasopharyngeal) depending on how the test is designed. It typically takes a longer amount of time to get results from a molecular test back than an antigen test. When tests are sent to a lab to be processed, it can take on average 1 - 3 days, and sometimes more if there are many tests to process. Some at-home tests and places with labs on the site of testing can provide results the same day, with some taking as little as one hour. The LAMP test is a specific molecular test that is faster than others, such as the PCR test, and is an example of a rapid molecular test. 

The main benefit of this type of test is that it is typically highly accurate, particularly in terms of catching positive cases (and as a result, not having many false negatives). As a result, molecular tests usually do not need to be repeated. In addition, molecular tests can be easily changed as the COVID-19 virus evolves, and hundreds of test samples can usually be run at once making the test process easier. 

Some of the negatives of molecular tests are the longer amounts of time it typically takes to get results compared to antigen tests and the many resources it requires (trained staff and special equipment are needed).

Antigen tests:

Antigen tests, which differ from molecular tests, look for “protein markers” on the outside of the virus. “Antigens” are small proteins that make up the virus. Antigen tests look for these proteins as evidence that the COVID-19 virus is present and being replicated in the body. Specific types of antigen tests include lateral flow tests (LFTs), which are also sometimes known as rapid antigen tests. 

Antigen tests are usually performed using a nasal swab (samples collected from the nasal walls of your nostril) or a nasopharyngeal swab (samples collected from deeper in the nasal cavity). Getting results from these tests can be very quick. Most of the currently approved tests return results in about 15–30 minutes. 

The quick time it takes to get results is one of the benefits of these diagnostic tests. Other benefits of antigen tests include the fact that antigen tests tend to be cheaper than molecular tests, and the fact that positive results are usually very accurate. Some of the negatives of this type of diagnostic test include the fact that false positives can happen, especially in areas where very few people have the virus and among infected people who are producing low levels of virus proteins. 

Additionally, these tests are less sensitive than molecular tests, meaning negative results may need to be confirmed with a molecular test, particularly among people with virus symptoms. The U.S. Centers for Disease Control and Prevention (CDC) advises people who show COVID-19 symptoms and test positive with an antigen test to get a PCR (molecular) test to confirm results.

Antibody tests:

Antibody tests -- an overall different category from diagnostic tests -- can tell you if you’ve been infected by the virus that causes COVID-19 in the past. Your body makes antibodies when it fights infections like COVID-19, or when you get a vaccine, such as a flu shot or the COVID-19 vaccine. Antibody testing measures the antibodies produced by your body’s immune system. 

Your body makes different types of antibodies, such as immunoglobulin G (IgG), IgA, and IgM. These antibodies, especially IgG, can last for a long time in the body after an infection. The COVID-19 antibody test looks for the presence of IgG antibodies for the COVID-19 virus that may have developed after someone was exposed to or infected by the virus. If someone tests positive on an antibody test, it suggests the individual was exposed to the COVID-19 virus in the past, which suggests there was a prior infection that has resolved or is still resolving, as well as potential protection against getting infected again.

Antibody tests are usually performed by taking blood either through a vein or with a finger prick. Getting results can range from coming back the same day at some places with labs on-site, to 3 days or sometimes more when sent to labs for processing. Antibody tests can not be used to diagnose active infections and should never be used for this purpose. However, they can be useful for improving our understanding of how many people have been infected with COVID-19, which can help with contact tracing, general surveillance, and making policy decisions as we move towards herd immunity.

However, antibody tests should not be used to evaluate a person’s level of immunity or protection from COVID-19 by themselves, especially among people who have received a COVID-19 vaccine. More research is needed to understand how vaccines impact antibody tests as well as how long protection lasts. In addition, results can be interpreted incorrectly (for instance, a positive result doesn’t necessarily mean you were infected), and a positive test result can result in riskier behavior.

Since the start of the pandemic, there has been a lot of talk about COVID-19 testing. However, at different points, having access to testing and keeping track of the different types of COVID-19 tests has been difficult. This difficulty has been made more complex by the many different companies doing COVID-19 testing, and the new developments in testing, such as the increasing availability of at-home tests and molecular rapid tests. 

Breaking tests down into two main categories: 1) diagnostic and 2) antibody, and then breaking down diagnostic tests into two main categories: 1) antigen and 2) molecular, can help simplify the many types of COVID-19 tests.

Also of note is that The U.S. Food and Drug Administration (FDA) does not currently recommend that antibody testing be used following COVID-19 vaccination to evaluate a person’s level of protection against COVID-19.