How accurate are the rapid at-home COVID tests?

Now that 15-minute home tests are available, millions of people can get a quick reading on whether their symptoms are due to Covid-19 or something else.

The at-home tests, although very fast and convenient, are less sensitive than the tests available at most Covid-19 testing centers. The home tests are “antigen” tests, while the gold-standard tests available at medical facilities use another technology, RT-PCR. (I’ll explain a bit more about the technical differences at the end of this article.)

An at-home test is far preferable to leaving your home, possibly exposing others to the virus, and then waiting hours or overnight for test results. The question is, how accurate is it?

Very accurate, for the most part. The chance that you’ll get an incorrect reading from a rapid antigen test is less than 1%. But (there’s always a “but”) it depends on what you mean by accurate. By another measure, they are not quite so reliable.

Let’s dig into the numbers from two new studies, which looked at thousands of cases, and see what they tell us.

In the first study, published recently in JAMA, Joshua Gans and colleagues from the University of Toronto looked at over 900,000 rapid-antigen test results. As expected, the vast majority of the tests were negative, and only 1,322 (0.15% of the total) were positive. They collected results using more-sensitive PCR testing for 1,103 of the positive tests, which allowed them to check whether or not the rapid antigen tests were correct. (This assumes the PCR test is always right, which isn’t quite true either, but it’s a good approximation.)

Surprisingly, 462 (42%) of the positive results were negative when double-checked with PCR. In other words, these were false positives. The investigators tracked these tests back to the source, and they discovered that about 60% of the false positives all came from a single batch of the Abbott Panbio Covid-19 rapid tests. So apparently there was a lower-quality batch that yielded more false positives.

However, another way to look at this data is that the overall rate of false positives was still very low. Out of more than 900,000 tests, only 462 were false positives, which yields a false positive rate of just 0.05%.

Another way to explain this is: if you just walked in off the street, your chance of getting a false positive test was about half of 1%. But once you saw that your results were positive, the chance that the positive result was correct was only about 60%, at least in that study.

One thing lacking in the Toronto study is that the investigators didn’t test everyone with PCR. They only used PCR to double-check the positive tests, so that study doesn’t answer the question of how many infections might have been missed.

Fortunately, the second study answers that question.

In this study, released in late January as a preprint on medRxiv by a group of my Hopkins colleagues led by Zishan Siddiqui, the investigators looked at 1054 participants, and tested all of them with both a rapid antigen test and RT-PCR.

Even though this study looked at far fewer subjects (1000 versus 900,000), they checked everyone with PCR, which allowed them to measure both sensitivity and specificity; i.e., they could count how many infections the antigen test missed.

So how good was the rapid antigen test in this study? First, its sensitivity was 92.7%, meaning that it correctly identified 92.7% of people who had Covid-19, whether or not they had symptoms.

What about those false positives? Here the news was better than the Toronto study, but still far from perfect: about 28% of the positive results from the antigen test were false. That’s better than the 42% found in the Toronto study, but it still means that many positive results from the rapid tests turn out to be incorrect.

What’s the take-home message from these studies? Well, I’d summarize it in three points:

  1. The rapid antigen test is generally very accurate, and certainly worth taking if you have any reason to think you might have Covid-19. The chance that you’ll have an incorrect reading, either positive or negative, is very small, less than 1%.
  2. If you get a negative test, you can relax: over 99.5% of negative results are correct, meaning you truly don’t have Covid-19.
  3. If you do get a positive test, you probably have Covid-19, but there’s still a roughly 30% chance that you don’t. If possible, you should immediately get a followup test using RT-PCR, which is more accurate.

Addendum: for those who want to understand the difference between the rapid antigen test and RT-PCR, here’s a bit more on those.

The rapid antigen test contains molecules called antibodies that bind to a specific molecule, the nucleoprotein, which is present on the surface of the SARS-CoV-2 virus. The antibodies in the test kit are designed to bind to other molecules that create a small band of color on a test strip, so you can see the results as a colored band in just 15 minutes. The main drawback of these tests is that they sometimes fail to detect the virus. For more details, a good description can be found here.

The RT-PCR test detects the RNA that is the genetic code of the SARS-CoV-2 virus. Every virus particle contains this RNA, which is a sequence of about 30,000 “letters” or nucleotides. In RT-PCR tests, we first convert the virus’s RNA to DNA, and then amplify it to make millions of copies. Because this test uses an amplification step, it can detect tiny amounts of virus, which is why it is more sensitive than rapid antigen tests. However, it takes at least a few hours to run this test, so it’s not as fast as the antigen test. For more details about how these tests work, check out the NIH explanation here.

COVID-19 probably started in a live animal market rather than a lab. That's still a huge problem.

A couple of weeks ago, three scientific studies were released on preprint servers (meaning they haven’t yet been peer-reviewed), all with the same conclusion: Covid-19 started in the Huanan Wholesale Seafood Market, in Wuhan, China.

I looked at the papers, and I’ll summarize them at the end of this article, but let’s start with the implications. For two years now, the world has speculated on whether the SARS-CoV-2 virus leaked from a lab. As I wrote in this space last June, and as many others have observed, the Wuhan Institute of Virology (WIV) was a possible source of the virus. WIV was doing research on coronaviruses in bats, including “gain-of-function” research that aimed to give viruses new capabilities.

It’s easy to imagine that an employee of WIV accidentally got infected, and unknowingly started a worldwide pandemic.

This “lab leak hypothesis” was fiercely denied by China, and initially denied by many independent scientists as well, who published statements that the virus almost certainly originated in the wild, probably in bats. Unfortunately, China shut down nearly all access to WIV, and probably destroyed all samples of coronaviruses there (we may never know), so it was impossible for the World Health Organization (WHO) or anyone else to conduct an independent investigation.

A year or so into the pandemic, as China continued to stonewall any investigation, scientists started to re-consider the lab leak hypothesis. It seemed highly plausible, for a number of reasons, that a lab leak could have occurred, even if no gain-of-function research was going on.

The three new studies, though, all point their fingers to the same place: the Huanan Seafood Market. All of them conclude that the virus probably first infected people directly from animals being sold, either for food or for use in Traditional Chinese Medicine, in the live animal market. Despite its name, incidentally, the market sold a wide variety of live animals, including raccoon dogs, bats, chickens, ducks, snakes, badgers, giant salamanders, and crocodiles.

Let’s suppose the new studies are right. Does this absolve China of responsibility for the Covid-19 pandemic?

Not at all.

China’s huge, open-air live animal markets have been implicated for decades in outbreaks of new infectious diseases, and China has known about it all along. The original SARS outbreak in 2002, which started in Guangdong province, was one prominent example. We were able to contain that outbreak because infected people were highly symptomatic, allowing public health experts to identify and quarantine them. At the time, the Chinese government suppressed reports of the disease and was slow to cooperate with the WHO.

Since at least the late 1990s, influenza experts have warned that new strains of bird flu could jump from birds to humans, and that China’s live animal markets were the most likely source. Many cases of bird flu were reported in the period from 1997 through 2005, often associated with live poultry markets, and yet China did nothing to regulate or shut down those markets.

Studies published in the early 2000’s showed that multiple animals, not just birds, were carrying the original SARS virus, and that these animals were being traded in China’s live animal markets. Yet China ignored the risks.

One reason these markets are especially dangerous to the public is that traders often sell unusual wild animals there, not for food, but for Traditional Chinese Medicine, as I’ve written before. Bats are among the many animals used (or abused) in TCM, and scientists reported as early as May 2020 that bats being sold in the Huanan Seafood Market were the most likely source of the Covid-19 virus.

China should have shut down these markets years ago. They represent an enormous public health threat, and yet the Chinese government ignored the warnings for years. Now that Covid-19 is winding down, or at least moving to an endemic phase, China has a new opportunity to show the world that it takes this threat seriously, by ending all trade in exotic live animals, and by shutting down its live animal markets. (Some live animals, such as fish, are not a threat and might be sold safely, but almost all mammals and birds carry viruses that could jump to humans.)

And lest anyone think otherwise, we still need to impose a ban on gain-of-function research in deadly viruses, as I’ve been arguing for years. All the way back in 2014, I asked the question, “should scientists be artificially mutating viruses so that they have the potential to become a worldwide pandemic?” I thought the answer was obvious, and yet the NIH (in the US) allowed the research to continue, after “pausing” it for a few years in the mid-2010’s.

When NIH initially paused its support for gain-of-function work, many virologists pushed back, arguing that they had plenty of safeguards in place, and no virus would ever leak out. Plus, they argued, their work was important in helping us prevent pandemics. Right.

We now know, definitively, that those virologists were wrong. We know that leaks are at least a possibility, even from the most secure biological labs, which is why multiple scientists, including some virologists, called for a fuller investigation into the lab leak hypothesis back in May 2021.

We also know that gain-of-function research did nothing to prevent the Covid-19 pandemic. Nothing, Zip. Nada.

We can take at least two lessons from this pandemic: first, that live-animal food markets should be shut down, especially those that sell wild animals rather than farm-raised ones; and second, that gain-of-function research on deadly viruses should be shut down as well.

Finally, for those who want to know about those three new papers I referenced at the beginning, here’s a brief summary.

The first paper, by George Gao et al., is a summary of extensive virus testing at the Huanan Seafood Market, on samples collected in early 2020. They found SARS-CoV-2 in 73 environmental samples (collected from locations in the market but not in people or animals), and yet “no virus was detected in the animal swabs covering 18 species of animals in the market.” In other words, the virus was found at the marketplace, but not in any of the animals, suggesting that infected people walking through the market were the source of those positive samples. Where did those people get the virus? This paper doesn’t answer that question.

The paper by Michael Worobey et al. shows more, however: not only do they show that all of the early cases were clustered in or very near the Huanan Seafood Market, but also that “positive environmental samples were strongly associated with vendors selling live animals.” In other words, even though we haven’t found the original animal source of the virus, the locations are all centered on live animal vendors. They conclude that the Huanan market was “the unambiguous epicenter of the COVID-19 pandemic.”

The third paper, by Jonathan Pekar et al., reports that there were actually two distinct events where the original SARS-CoV-2 virus jumped from animals to humans, both at the very beginning of the pandemic, probably in late November 2019. These authors also point the finger at the Huanan Seafood Market, and they implicate another animal, raccoon dogs, as a likely source. Raccoon dogs are closely related to foxes (not raccoons, despite the name), and were being sold as food in the Huanan market. Scientists have known since at least 2003 that raccoon dogs can carry SARS coronaviruses. This paper illustrates yet again why live-animal markets represent a threat to human health.