Breaking down COVID-19 tests: what’s out there and what’s the difference?
You may or may not know that the FDA has issued over 200 different emergency use authorizations (EUAs) to various manufacturers for COVID-19 tests. While each of these tests broadly falls into two main categories, the differences between and within these categories can be a confusing mess of scientific jargon.
If you’re a reader in the United States, chances are that you don’t have a lot of freedom to go out and choose the COVID-19 test that you like best, but this article will attempt to give you a better understanding of the pros, cons, and use cases for different tests as the pandemic continues to develop.
Sensitivity vs Specificity
Before we dive into the different types of tests, let’s talk about how we can compare their performance. Sensitivity and specificity are two commonly used metrics for medical tests, and basically describe how likely a test is to correctly identify positive and negative subjects. A test with high sensitivity will be positive for most people with the disease whereas a test with high specificity will be negative for most uninfected individuals. A “good” test will generally perform well in both of these areas, but you might imagine cases where one or the other is more important. High specificities will ensure that you are not forcing people to quarantine who do not have the virus. High sensitivities ensure that you don’t incorrectly tell someone they are disease-free when they are actually infected.
Diagnostic Tests
As the name might suggest, diagnostic tests are meant to diagnose patients with COVID-19 (or, perhaps more accurately, diagnose people with infection by SARS-CoV-2, the virus causing COVID-19). Ultimately, these tests attempt to identify some part of the virus in patient saliva, blood, or mucosal specimens. These tests can be broken down into two more categories based upon the part of the virus being detected; molecular tests detect viral RNA and antigen tests detect viral proteins.
Molecular Tests
The most common type of molecular test makes use of RT-PCR (reverse transcription polymerase chain reaction) to detect viral RNA and is most commonly used to diagnose people using specimens obtained via nasopharyngeal swab. These tests are the “gold standard” in COVID-19 testing, and are both highly sensitive and highly specific. However, they are also generally more expensive, take longer to receive results, and are fairly inconvenient. Newer technologies are on the horizon in this area, including RT-LAMP (reverse transcription loop-mediated isothermal amplification). These approaches promise to be cheaper and faster than RT-PCR, but tend to be less sensitive.
Antigen Tests
Antigen tests aim to detect viral proteins in patient samples and comprise a diverse group of testing approaches. These tests are cheap (as low as 5$), fast, and do not rely on specialized laboratories to process results. Unsurprisingly, they are also less sensitive and specific when compared with RT-PCR, and are less useful in diagnosing people with low viral loads (usually when a patient is in the very early or late stages of infection). These characteristics make the test appropriate for “point of care” (POC) testing when results are required quickly or frequently. Further, some believe that these tests might be the key to controlling outbreaks of COVID-19 in the US, and their price makes them attractive for high-frequency public policy testing frameworks.
Serological (Antibody) Tests
From the standpoint of the FDA, “serological” and “antibody” can be used interchangeably to describe the class of tests detecting antibodies in the serum (blood) of infected or recovered COVID-19 patients. These tests are not to be used diagnostically, if possible. Antibodies are produced by an individual’s immune system to fight infections, but may not be detectable by tests until up to weeks after infection. Instead, these tests are often used to identify people who might have previously been infected by COVID-19 (antibodies can remain detectable for up to months following infection).
The tests I have outlined above constitute the majority of modalities that you might encounter currently in the United States. If you are able to choose the type of test you receive, you are now equipped a little better to make that decision. Furthermore, as more tests become available and as the importance of testing grows in public health efforts to combat the COVID-19 pandemic, you are now prepared to navigate the many different testing policies and frameworks that may be suggested in the coming months. For a more in-depth overview of the scientific articles available concerning different COVID-19 tests, please see this blog.
