The COVID-19 pandemic has sparked a great testing debate in this country, and you’ve likely heard a lot of terms thrown around: RNA tests, antibody tests, point of care tests, laboratory tests, etc. But what are the differences between them? When is each one useful? Not every test gives your healthcare provider the same information, and not every test is useful at every stage of an infection.
In this post we will break down three things: the types of tests out there, what information each test can give us, and when that information is useful. There are a lot of good explainers on this if you are interested in diving into the details, and some great resources are listed at the end of this article for more info!
Let’s start by discussing the types of tests available. First, there are the nucleic acid tests that detect DNA and RNA. The SARS-CoV-2 virus that causes the COVID-19 disease is an RNA virus, which means that its genome is made up of RNA instead of DNA (like ours!). The RNA sequence is specific to that virus, and detecting its presence tells us if someone has been infected with the virus. This test will read as positive when the viral RNA is present, which can occur as soon as the virus is replicating in your body and until your immune system has completely cleared the virus. The amount of virus in your body is roughly proportional to the amount of viral RNA, since each viral particle contains a single RNA genome. Therefore, the amount of viral RNA in your body will change over the time that you are sick, and will go away when you are well and the virus is gone.
So, a test for the RNA, or a “nucleic acid test”, will be able to tell you when you are infected with the virus. The test will be accurate at any stage of infection as long as you have enough virus in your body. Whether you have been infected but aren’t showing symptoms, you’re feeling sick, or you’re starting to feel better, the nucleic acid test can still be positive if there is enough virus in your system. Viral load, or the amounts of virus in your body, tends to follow a pattern that starts low, peaks or levels off, and finally falls off as you get better. The amount of virus can vary from person to person at each stage of an infection. Factors like how old you are or if you are immunocompromised can alter how much virus is in your body during an infection. The variables that affect viral load for this COVID-19 disease are not yet well known. Learning more about these factors will help us get better at knowing when to test, who to test, and how to make better tests.
The caveat about nucleic acid tests is that in most cases, they require a sophisticated laboratory set-up to perform. This is one reason why it was difficult to ramp up COVID-19 testing in the United States. Not every lab is designed for this kind of work, which limits how many tests can be performed in a day. However, RNA tests can also be done on smaller test platforms in less sophisticated labs. These are called point-of-care tests, or POC tests. Recently, Abbott released a small scale POC RNA test for flu and other infections which interfaces with a testing system they already have in the field. The test can produce results in about 15 minutes, but only one test can be run at a time, while more sophisticated labs can run hundreds of tests or more at a time. There are about 18,000 of these instruments already in doctors offices and small labs around the country, and Abbott claims they can ship 50,000 test cartridges a day for these systems.
Unfortunately, for a number of reasons, the United States has not invested the necessary capital and resources in POC nucleic acid testing and research. The technologies to make RNA testing portable and accessible do exist, but the cost/benefit ratio has not been favorable in the US — until now. During a pandemic, when lots of people need tests, the economics do work. The conundrum, however, is that in normal circumstances, when a small doctor’s office is running only a few tests a day for different infections, it often does not make sense for them to have this testing on site. So, they use large lab testing companies instead. Samples from the practice are picked up like the mail by couriers and driven to central facilities for testing. The results are returned to your provider electronically.
The COVID-19 pandemic has cast POC testing into the spotlight, making it evident just how essential it can be for disease containment. POC tests are often touted as a solution to healthcare inaccessibility in low resource settings, but the reality is that POC tests are vital to all settings. As more novel infectious agents emerge, we hope that even high resource countries will prioritize POC development.
When you first think of POC testing, what comes to mind? Many people think of urine-based pregnancy tests, which are widely available at drugstores. A pregnancy test detects a specific protein in urine called HCG, which is present in high amounts during pregnancy. If significant amounts of this protein are present, the test shows a positive result.
For infectious diseases, these types of tests are often called “antibody tests.” When you contract a virus, your immune system will attack it, creating special proteins called antibodies in the process. These proteins bind to the virus so immune cells can locate and destroy it. The body creates unique antibodies for each pathogen it encounters, so they can be used as indicators of past or present infection. For example, if your blood contains antibodies to tuberculosis, you must have been exposed to tuberculosis at some point. These antibodies hang around in your system so that if you encounter the same disease again, your body is prepared to fight it off.
Compared to RNA tests, these kinds of antibody tests are fast and relatively simple, but they can produce false-negative results early in the disease course. For antibody tests to be effective, you need to have been sick long enough to produce the antibodies, otherwise the test will appear as a false-negative. In the setting of a pandemic, this can be particularly problematic, as false-negative results may prevent infected individuals from taking the proper isolation precautions. Furthermore, antibody tests can also give false-positive readings if you have already recovered from the infection, as the antibodies remain in your system long after symptoms resolve. Antibody tests to detect COVID-19 will likely use blood from a finger stick. The healthcare company Henry Schein has announced the availability of a new point of care antibody test that works on this principle.
Nevertheless, antibody tests have a very important role to play in this pandemic. As more people recover after being sick, we will need to know who those people are because they will have developed some amount of immunity to the disease. They will likely be able to move freely through an infected population without becoming infected again. However, it is important to note that while this is true for many viral infections, it is not yet known if COVID-19 will behave exactly the same way.
There is a second kind of antibody test that has the potential to work earlier in the course of the disease. These tests use antibodies to look for proteins on the outside of the viral envelope. We will call these viral-protein tests. For example, E25Bio, a Cambridge-based biotech company, has developed a POC test for COVID-19 that is currently being tested at Massachusetts General Hospital. The test resembles an over-the-counter pregnancy test, with one line vs two lines in the readout window. This new test, named the “Spike Dart,” can detect viral proteins in various bodily fluids, including mucus, saliva, blood and urine. The Spike Dart provides results in about 15 minutes. However, the tradeoff for quick results may be decreased sensitivity compared to the lab-based PCR assays, which is common for point-of-care tests. Nonetheless, this POC test may be a valuable tool in healthcare settings to rapidly isolate infected individuals for further testing and treatment. E25Bio has been in close contact with the FDA and hopes to attain emergency use authorization to deploy the Spike Dart in hospitals and doctors offices in the coming weeks.
To summarize, there are three different types of tests in the news right now: RNA (nucleic acid) tests, antibody tests that look for antibodies against the virus in the blood, and viral protein-based tests that use antibodies to detect viral proteins. RNA tests are good at detecting new infections and monitoring how long a person is infected. Antibody blood tests can be more easily made portable to use in the field, but can only detect late-stage disease and past infections. Viral-protein POC tests are simple, fast, and inexpensive, but may operate at a lower sensitivity than lab-based testing.
This post was updated on 4/1/20 to include information about viral protein detection tests.
How Covid-19 Immunity Testing Can Help People Get Back to Work
Coronavirus Testing Shouldn’t Be This Complicated
Current and Future Applications of Point-of-Care Testing – Centers for Disease Control