This article was originally published April 29, 2020 and has been updated by the author.

If you've learned one thing about laboratory testing for COVID-19 in the past couple of months, it's probably that "it's complicated." If you're paying close attention, you might also know that it's not a one-size-fits-all approach. As new tools, like antibody and rapid antigen testing become readily available, new questions continue to arise. It's important that we understand the value and limitations of COVID-19 testing so that we can best use the current (and developing) items in our toolbox to effectively control the spread of SARS-CoV-2.

What Types of COVID-19 Tests Are Available?

At this time, there are 3 types of tests that have been authorized by the U.S. Food and Drug Administration (FDA) for the detection of COVID-19:

• Rapid antigen tests, also known as point-of care tests.
• Molecular tests, also known as nucleic acid amplifiction, genetic, RNA or PCR tests.
• Antibody tests, also known as serology tests.

Rapid Antigen Testing (Point-of Care)

How Are Rapid Antigen Tests Performed?

Rapid antigen tests are immunoassays that require respiratory specimens (typically nasopharyngeal or nasal swab) from the patient because SARS-CoV-2 is a respiratory virus. Once collected, samples are placed in an extraction buffer or reagent and tested for the presence of SARS-CoV-2-specific antigens.   

What Does Rapid Antigen Testing Reveal?

Rapid antigen tests tell us if a patient is actively infected with SARS-CoV-2. They detect the presence of virus-specific proteins, called antigens, from patient specimens. As of Aug. 18, 2020, the FDA has issued Emergency Use Authorization (EUA) for 3 rapid antigen tests. All of these detect SARS-CoV-2 nucleocapsid protein (N) antigens. Results are produced quickly, generally within 15 minutes, and can be performed near the place and time of patient care (as opposed to a separate laboratory setting). If the viral antigen is detected, it indicates that the virus is present.

Is There an Accuracy Concern with Rapid Antigen Testing?

These tests are generally less sensitive than molecular tests, and it's important to be aware that there is an increased possibility of false negatives with rapid antigen testing. They are most accurate when viral loads are high (within the first couple of symptomatic days), and your healthcare providor may order a molecular test to confirm the results of your rapid antigen test if it indicates that you are negative for SARS-CoV-2. If your rapid antigen test is positive, there is no need to confirm with a molecular test, as these tests are highly specific.

Molecular Testing (Nucleic Acid Amplification)

How Are Nucleic Acid Amplification Tests Performed?

Nucleic acid amplification testing requires respiratory samples from the patient because SARS-CoV-2 is a respiratory virus. Nasopharyngeal swabs are most commonly used. Lower respiratory secretions, such as sputum and bronchoalveolar lavage fluid, are also used if a patient has pneumonia or lung involvement with infection. 
 
Samples are then processed and tested for SARS-CoV-2 RNA. The test includes extraction of RNA from the patient specimen, conversion to DNA and PCR amplification with SARS-CoV-2-specific primers.

What Does Nucleic Acid Amplification Testing Reveal?

Nucleic acid amplification tests tell us if a patient is actively infected with SARS-CoV-2. They detect the presence of characteristic sequences of SARS-CoV-2 genetic material (RNA) in respiratory samples of patients. If the viral RNA is detected, it suggests that the virus might be present.

What to Do with a Positive Nucleic Acid Amplification Test?

If you test positive for SARS-CoV-2, you should self-isolate, call your doctor if your symptoms worsen and seek medical help immediately if your condition becomes critical. 

Is There an Accuracy Concern with Nucleic Acid Amplification Testing? 

Molecular tests can yield a false negative result if the level of viral RNA in a particular sample is too low for detection, and results can be skewed if steps are not taken to ensure that the tests are performing properly in the first place. The American Society for Microbiology has developed step-by-step verification procedures to help labs develop efficient and effective verification protocols for commercial EUA COVID-19 molecular tests. These procedures will help ensure that data is accurate.

Comparison of COVID-19 Molecular and Antibody Tests

Antibody Testing (Serology)

How Are Antibody Tests Performed?

The immune system naturally produces proteins, called antibodies, in response to infection. Antibodies recognize and bind to foreign molecules, called antigens, on the surface of pathogens. When an antibody binds to an antigen, it seeks to disable the associated pathogen through direct or indirect mechanisms. 
 
Blood samples are used as the specimen type for antibody testing. SARS-CoV-2-specific antigens (SARS-CoV-2 spike or nucleocapsid proteins) are mixed with the patient's blood, and if antibodies to SARS-CoV-2 are present in the blood, the two will bind to one another, resulting in a color change that marks the test positive.
 
It's important to recognize that this test measures the amount of antibodies produced in response to SARS-CoV-2 infection and does not test for the presence of the virus itself. Therefore, it does not indicate if a person is currently infected.

What Does Antibody Testing Reveal?

Antibody tests tell us if a person has been exposed to a virus or other infecting microorganism. They detect the presence of antibodies that are produced in response to infection. Antibody tests will also identify if a person has been exposed to SARS-CoV-2 and developed immunity against SARS-CoV-2 infection.

How Long Does It Take for Antibodies to Develop? 

According to the CDC, it typically takes 1 to 2 weeks for a patient to develop antibodies to SARS-CoV-2. Because of this delay, antibody tests are not currently recommended for the diagnosis of acute cases of COVID-19.

Can Antibody Testing Be Used to Help Determine Who Is 'Safe' to Return to Work?

Antibody testing reveals if a patient has been exposed to the virus but does not tell whether an active infection occurred, or whether the antibodies produced are the kind that can prevent another infection. Furthermore, it does not reveal how long protective immunity to SARS-CoV-2 infection will last if developed. If we can determine those things, they will help inform decisions about the safety of returning to normal activities.

Is There an Accuracy Concern with Antibody Testing? 

Antibody tests can yield a false negative result if the test is conducted too early and antibodies have not yet developed. They can yield a false positive if antibodies to coronaviruses other than COVID-19 are present (e.g., SARS-CoV and MERS-CoV). It is therefore important that serology tests are carefully designed to be specific for SARS-CoV-2 and are appropriately validated and approved for COVID-19 testing. The American Society for Microbiology has developed step-by-step verification procedures to help with this.

What Does the Future of Antibody Testing Look Like?

Broad use of validated (or regulatory-approved) serological tests can give a big picture of the number of people who have been infected or exposed to SARS-CoV-2 and help to generate surveillance data as we try to control the spread of COVID-19. The authorization of complete home tests for antibody testing is under discussion by federal authorities and test developers.