At the moment, there are two main types of tests for Covid-19:
1. PCR Test: Tests for a current infection – at the moment, this is the most reliable test to detect whether a person is currently infected with the virus
2. Antigen (Rapid) Test: Tests for a current infection – this is a rapid, but relatively unreliable test to detect whether a person is currently infected with the virus
3. Antibody Test: Tests for a current and past infection – this test primarily tells you whether a person was previously infected with the virus or is in the later stages of an ongoing infection
Here is their main overview and comparison:
PCR TESTS
ANTIBODY (SEROLOGICAL) TESTS
How it works:
Testing for the presence of the virus’ genetic material from a nose or throat swab. It is done using a technique called reverse transcription polymerase chain reaction (RT-PCR) This type of assay detects and amplifies a specific genetic sequence in the virus. This method is highly sensitive and specific, but can produce false positives if lab reagents become contaminated.
Testing to detect past viral infections by looking for viral proteins or antibodies the body has produced to fight the virus. There are several options (both rapid tests and ELISA). The test is performed on a blood sample and the antibodies that are detected are immunoglobulins M and G (IgM / IgG), proteins produced by the person’s immune system against the coronavirus. IgMs are the antibodies that are produced early during infection and IgGs appear late, persisting over time. The rapids test for antibodies are done with a device called lateral flow assay, which is essentially a dipstick encased in a cassette (just like a home pregnancy kit.)
Technical requirements:
The swab can be taken from the patient anywhere but it then needs to be processed by a special PCR machine in a lab called thermocycler. They can be performed on open (“manual”) or closed platforms (that is, the kits only work on automated, proprietary systems).
The test can be carried out by the patient at home using a small device called a lateral flow assay. It normally does not require specialized lab equipment to process.
How long does it take to get the result:
The entire process takes from a few hours to a few days (including delivery of the samples to the lab.)
The results can usually be obtained in minutes.
When does it work:
The test can detect the virus within 24-48 hours after the patient was infected. However, PCR no longer works when the patient has fully recovered, i.e. when the virus is no longer present in the body. It will not detect a resolved infection.
The test only works after the patient’s body has been infected for sufficiently long to develop its own antibodies against the virus. This can be many days from the moment of initial infection. However, it also works after the patient has recovered, for months and potentially even years. But the dynamics of antibody response and production during the different stages of infection are not yet fully established, therefore these assays should not be used to rule out a case during the first days of illness.
Main advantages:
Can detect the presence of the virus early into the infection even when the patient is in the incubation period and does not yet show any symptoms.
Produces results even when the patient is no longer sick, which is critical for identifying who has acquired immunity against the virus. This is especially important given that many Covid19 cases appear to be asymptomatic, and the patients did not realize that they were infected. Assays based on the detection of antibodies can support outbreak investigation and contact tracing.
Typical cost per test:
Tens to hundreds of USD
Single digits to few tens of USD
THE ANTIGEN (RAPID) TEST is used for the same purposes as the PCR test, i.e. identifying a current infection. It is based on detecting the virus proteins (antigens) in blood or respiratory secretions using the same technology as most of the antibody tests – the lateral flow assay. That makes it less reliable than the RT-PCR test, but allows to speed up the confirmation of suspected cases. The probability of correctly diagnosing a person with Covid-19 (sensitivity), depends on the phase in which the patient is, since the viral load (the amount of virus present in the human body) varies throughout the different stages of infection. When symptoms are present, the sensitivity is higher than 90%, but without symptoms it drops to 60%. Due to this limitation, it is not considered a definitive diagnostic test, but it’s useful for screening purposes. In doubtful cases, it is necessary to perform the RT-PCR test or repeat the antigen test later (when there are more symptoms) to confirm or rule out the presence of coronavirus.
READ ALSO:
- Navigating the Wild West of COVID-19 antibody tests
- Fast, portable tests come online to curb coronavirus pandemic – a detailed article on Covid-19 testing technologies in Nature
- Latest WHO guidance on Covid-19 tests and testing
- UK medicines watchdog warns over unsafe coronavirus tests
- Getting our life back and saving the economy with a coronavirus green card