Most tests for COVID-19 involve taking a swab sample from the back of the nose, throat, or both. But some tests require a blood sample.

These tests aim to find out if a person has, or has ever had, an infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This is the virus that causes coronavirus disease 19 (COVID-19).

A molecular or antigen test can tell whether a person currently has the infection, regardless of whether they have any symptoms.

A serological, or antibody, test shows whether the body has antibodies to the virus. It cannot show whether a person has an active infection. If antibodies are present, it suggests that the body has already dealt with SARS-CoV-2.

Currently, more than 415,000,000 tests have been carried out in the United States. More than 8.23% of the results have been positive.

This article will explore how COVID-19 tests work, which ones are available, when to have a test, and how to find one.

a medical professional storing a swab used for coronavirus testingShare on Pinterest
A test for a SARS-CoV-2 infection often requires a swab sample.
Image credit: Chip Somodevilla/Getty Images.

There are several ways to test for evidence of the virus. A person may have one of the following types of COVID-19 test.

Molecular tests

Molecular tests look for signs of a current infection. One type of molecular test for the virus that causes COVID-19 is a real-time polymerase chain reaction (RT-PCR) test. This is sometimes called a nucleic acid amplification test.

A person performing a molecular test usually takes a sample from the back of the throat, the nose, or both using a long cotton swab. A person might take their own swab sample at home.

RT-PCR test

This involves a healthcare professional collecting a sample and sending it to a laboratory for testing. The analysis can detect signs of the virus’s genetic material in the sample.

This test can confirm a diagnosis of COVID-19 if it identifies two specific SARS-CoV-2 genes. If it identifies only one, it will produce an inconclusive result.

Antigen tests

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Ways of taking a swab for a COVID test: Check the test instructions to find out which location is recommended.
Illustration by Diego Sabogal

An antigen test is sometimes called a lateral flow test. It detects proteins produced by the virus. This is a rapid test that people can take at home using a small device that resembles a home pregnancy test.

After taking a sample using the swab provided, the person dips the swab in a vial of fluid. They then insert the swab, which now contains some fluid, into a small device. As the fluid spreads through the device, a line should appear in a screen on the device. This is a control line that shows if the test is working correctly.

After some time — maybe 15 minutes, depending on the specific test — another line may appear. If this happens, the result is positive for the virus. If the second line does not appear, the result is likely negative.

Molecular tests can only help diagnose current infections. They cannot tell whether someone has had COVID-19 in the past and recovered.

Serological tests

These detect antibodies the body produces to fight SARS-CoV-2. The antibodies are present in blood and tissues of anyone who has recovered from COVID-19.

Serological tests require a blood sample. While they are useful indicators of a past SARS-CoV-2 infection, these tests may also detect antibodies produced to combat other types of coronavirus. So if the result is positive, it does not necessarily mean that the person has had COVID-19.

In the U.S., tests are available at health centers and some pharmacies in every state. Some areas have dedicated testing centers. Schools, workplaces, and other organizations may also have testing programs.

The tests themselves are free of charge to everyone, regardless of their insurance status. However, if a person gets their test as part of a doctor’s visit, there may be costs.

Some people require a prescription for a test, and some tests are only available in certain locations. An at-home test may produce a result right away, or a person may have to send their sample to a laboratory for analysis. In some cases, a collection service is available.

Rapid tests, also called direct-to-consumer or over-the-counter tests, are also available for purchase online, often without a prescription. Make sure that any such test has authorization from the Food and Drug Administration (FDA).

Not all tests are suitable for everyone. Contact a healthcare professional or visit the local health department’s website to learn about the available options and confirm which type of test is appropriate.

Learn more about at-home testing options here.

No test is 100% reliable, as the FDA note, and it is possible to have a false-negative or false-positive result. This may happen if the person takes the test incorrectly or takes it too soon after exposure to the virus.

If a person has a positive result, there is a high chance that they have COVID-19. They should take precautions such as isolating and monitoring for symptoms.

If the result is negative but the person has symptoms, they should also isolate. Their result may be a false negative.

Antigen tests may be less accurate than RT-PCT tests. The FDA point out that if a person has symptoms but gets a negative result from an antigen test, it may be a good idea to have an RT-PCR test and self-isolate.

If a person has a negative result and no symptoms, they do not need to isolate but should continue to wear a face covering in public and practice physical distancing.

Whether an antibody test result is positive or negative, continue to take precautions, such as wearing a face covering. The result does not guarantee immunity and does not mean that the person cannot pass on the virus.

The Centers for Disease Control and Prevention (CDC) recommend that if a contact tests positive for the virus, their contacts should stay home for 14 days after their last encounter with that person. This is not necessary, however, for people who are fully vaccinated and have no symptoms or people who have recovered from COVID-19 in the last 3 months.

As the CDC note, a person should consider testing if:

  • They have symptoms of COVID-19, such as a cough or fever.
  • They have been within 6 feet for a total of 15 minutes in any 24-hour period of someone who has tested positive.
  • They have been in a setting where exposure may have occurred, such as a crowded train.
  • Their healthcare department or another authority asks them to have a test.

Learn about the different types of vaccines and their possible side effects.

People do not need a test if:

  • They are fully vaccinated against COVID-19, even if they have had exposure to someone with the underlying infection.
  • They have tested positive and recovered from the infection within the last 3 months — unless they develop new symptoms.

What are the symptoms of COVID-19?

Access to testing facilities has expanded significantly in some countries over the last year. In the U.S., many testing options are now available.

The two main types are diagnostic tests, typically involving a swab, and antibody tests, which is usually require a blood sample. A diagnostic test can show if a person has a current SARS-CoV-2 infection, and an antibody test can show if they have had one in the past and recovered.

In the U.S., anyone who needs a test can access one for free. They can find out how to do this from their local health department.