Fighting Viruses: How Does Coronavirus Testing Work?

Accurate tests for the presence of the virus that causes COVID-19 are necessary to ensure patients get proper medical care and to better control the spread of disease in the community.

Multiple tests for SARS-CoV-2 are currently available, including at-home test kits.

Nucleic acid amplification tests (PCR tests)

Nucleic acid amplification tests are used to detect viral nucleic acids, which are present during and after an infection with a respiratory virus. They determine whether a person is currently or was previously infected. For SARS-CoV-2, this test identifies specific sequences of SARS-CoV-2 RNA, the virus's genetic material.

The nucleic acid amplification test, the test predominantly used during the early stages of the pandemic, typically relies on swabs from the nose and throat. Genetic material from these samples is amplified in the laboratory using a process called RT-PCR (reverse transcription polymerase chain reaction). If present, specific sequences from SARS-CoV-2 RNA will be detected proportionally to the amount of virus in the sample.

PCR testing can detect the presence of the virus early in the infection. However, the swab needs to pick up enough of the virus sample or a false negative will result.

PCR tests can also detect the presence of viral nucleic acid after the window when infectious virus is detectable. If a follow-up PCR test shows viral RNA is increasing, this implies an active infection; if viral RNA is decreasing, this implies the end of an infection.

Antigen tests

Like nucleic acid amplification tests, antigen tests can be used to diagnose an infection. They work by checking samples, often collected via nose or throat swab, for fragments of proteins found on or in viruses.

The main advantage of an antigen test for for SARS-CoV-2, the virus that causes COVID-19, is its speed. Results can usually be obtained within minutes; most rapid tests are antigen tests. Antigen tests also can be performed at home or at the same time and place where patient care is provided, unlike nucleic acid amplification test samples, which often have to be sent to analytical laboratories. In the U.S., at-home antigen test kits are readily available.

Antigen tests can generally be produced at a lower cost than nucleic acid amplification tests and thus may be scaled up more easily.

However, antigen tests are not as sensitive as nucleic acid amplification tests. This means that positive results from antigen tests are highly accurate, but there is a higher chance of false negatives.

The U.S. Food and Drug Administration (FDA) issued its first emergency use authorization for a COVID-19 antigen test on May 8, 2020. In January 2022, the federal government made free at-home antigen tests available to every home in the nation.

Antibody tests

Antibody testing, or serological testing, is used to determine whether a person has been infected by checking for antibodies against the virus. The presence of antibodies indicates that the person has been exposed to the virus and that their body has mounted an immune response against it. This test requires a blood sample and does not look for the virus directly.

Two types of antibodies, immunoglobulin M (IgM) and immunoglobulin G (IgG), are typically detected. IgM is the body's first type of antibody to react to the infection. Detection of IgM indicates that a person has been infected recently. IgG indicates that a person has had the virus in the past. If the person is exposed to the virus again, they may be able to fight it if IgG is present in their blood. We do not yet know how long these antibodies last in the blood or whether they provide protective immunity. Retesting people with antibodies against SARS-CoV-2 over time will provide more information.

Antibody tests can be conducted using point-of-care testing, meaning they can be performed at the same time and place a patient is being treated. The antibody test for coronavirus can be performed using a finger prick of blood, similar to the way a blood glucose meter is used.

Because antibody tests provide information about who has been infected in the past, they can provide useful insight into the pervasiveness of the disease in a community. Scientists do not yet know if people with COVID-19 antibodies are protected from the disease for any period of time.

The U.S. Food and Drug Administration (FDA) approved a test for antibodies against the novel coronavirus on April 2, 2020. Multiple tests have since been released.

Read more about disease modeling, another important tool for addressing the coronavirus pandemic, on the Caltech Science Exchange >