As Oregon’s academic health center, Oregon Health & Science University has developed extensive expertise in measures to treat and prevent the spread of COVID-19.
Testing does nothing to prevent an infection. That’s why it’s critical for Oregonians to continue to take the only measures proved to prevent the spread of the virus: limiting social gatherings, physical distancing, regular hand washing and consistently wearing facial coverings.
Yet testing does play a role in managing the pandemic, and people wonder about the various types of tests available.
There are two major kinds of diagnostic tests.
The first involves collecting saliva or nasal mucus to test for markers of the virus itself – an active infection. These tests can search for genetic material of the virus or a specific protein on the surface of the virus. Technology to collect samples, process results and improve accuracy is evolving rapidly.
The other major type of test searches for antibodies in blood samples. Antibodies can be elicited by the virus during a prior infection. This type of test may be useful as a way to track the spread of the virus across a large population, but its clinical value is limited.
What type of test is best?
The best way to think about testing is to consider why you need it. People may have no symptoms but worry they’ve been exposed to the virus, or they may want to rule out the chance of carrying the virus before boarding a plane or visiting a family member. Others are required to take a test before going in for a medical procedure, to reduce risk to health care workers. Still others are curious if they have had COVID-19 in the past.
In each case, it’s important to always consult your health care provider.
See highlights of the types of tests below:
Tests for active infection
1. Molecular
Purpose: Discover active infection.
Sample type: Saliva or nasal mucus.
Test turnaround time: Rapid versions of these tests exist that use specially manufactured cassettes and provide results within one to two hours. Other laboratory PCR, or polymerase chain reaction, tests can be conducted within a day or two, although commercial labs have sometimes taken a week or more due to a high volume of tests. At OHSU, the turnaround time is typically within 24 to 48 hours.
Accuracy: 85% to 98% depending on the sample type and process.
Collection methods:
- Saliva can be collected by spitting into a special type of collection tube.
- Nasal mucus is collected by a nasal swab that is inserted within the nose or a nasopharyngeal swab that is snaked deeper through the nose to the back of the throat. The deeper in the respiratory system the sample comes from, the more likely it is to pick up virus in the body.
Process: Technicians extract the relevant genetic material of the virus either through heating or a chemical reaction. Heating is less sensitive than extracting through chemical reaction. The sample then runs through a process called polymerase chain reaction, or PCR, to detect specific genetic material unique to the SARS-CoV-2 virus.
Limitations: Availability of testing supplies, including chemicals and swabs, has sometimes been limited. As with all diagnostic tests, the viral load within an infected patient varies over time. OHSU recommends testing no sooner than three to five days after exposure, when viral loads are typically high enough to be detected, after they have isolated themselves to the extent possible.
2. Antigen
Purpose: Discover active infection. The accuracy of the antigen test is best when the person is clearly ill.
Sample type: Saliva or nasal mucus.
Test turnaround time: 15 minutes to a day or two.
Accuracy: Varies. Depending on the amount of virus in the patient at the time of the test, and the process used to detect the antigen, the accuracy can be as low as 50% or as high as 95%.
Collection methods:
- Saliva can be collected by spitting into a special type of collection tube.
- Nasal mucus is collected by a nasal swab that is inserted within the nose or a nasopharyngeal swab that is snaked deeper through the nose to the back of the throat. The deeper in the respiratory system the sample is taken, the more likely it is to pick up virus in the body.
Process: Antigen tests detect a specific protein known to be on the virus surface. This can be done using a laboratory test or on site using a cassette. The cassette can provide results within an hour, similar to how a home pregnancy test would be used.
Limitations: Antigen testing has also been used for large groups of asymptomatic people as a fast-turnaround screening tool, although the test is not approved for use in this setting. The test has also not been approved for use in children. The risk of a false negative result is high, so people should continue to follow all public health measures to limit the spread of the virus – especially given that as many as 40% of people with COVID-19 have no symptoms at all. These people risk spreading the virus unwittingly. Sometimes, symptomatic or exposed people who test “negative” with an antigen test may be recommended to undergo repeat testing with a molecular test. The antigen test has not been approved for use in children.
Test for prior infection
Antibody test
Purpose: Discover prior infection. This result is helpful to calculate the prevalence of the virus in a given community. It also may be useful for health care providers to diagnose other illnesses in people who have symptoms that appear to be consistent with COVID, but the molecular or antigen tests are negative. It will also be important to determine immunity once a vaccine is widely available.
Sample type: Blood
Test turnaround time: Within a day.
Accuracy: Usually 98%.
Collection method: Finger stick or blood draw.
Process: Technicians using specialized laboratory equipment search for the presence of immune proteins called antibodies, ideally a specific type of antibody that blocks infection from the SARS-CoV-2 virus.
Limitations: Antibody tests vary in the types of antibody they can detect, and it’s unclear which type of antibody is needed to confirm that the patient had an infection. Therefore, a positive antibody test does not necessarily guarantee immunity from future infection with COVID-19. The reverse is also possible: Failure to detect antibodies in one test does not rule out the possibility of a previous infection.