Rapid COVID-19 Tests Can Be Useful – But There Are Far Too Few To Put a Dent in the Pandemic

Yves here. I am sure readers will have more to say about this article on rapid Covid tests, as in antigen tests, which require a non-traumatic swab of both nostrils (the PCR tests are the ones that sample the back of your throat via your nose). Two additional points: the antigen tests have high accuracy in symptomatic patients, but generate a lot of false positives otherwise. And they aren’t cheap. Here in Alabama, the charge at the urgent care center (which did test and process the results pronto) was $185, which they said my insurer would pay….but my reading of the fine print on Cigna’s site said I had to be recently exposed. Fortunately, that time, I had just been in the ER for four hours, and mistakenly spent about 20 minutes in the Covid section (no signs or partitions whatsoever; it was a guard who noticed and shooed me out). The bathrooms were also off the Covid area. So while I am presumably OK for this one test were Cigna to challenge me, how much tolerance would they have for paying for more tests, even though the level of uncontrolled spread in the US means that anyone who isn’t a hermit having everything delivered to them can contend that they’ve been exposed?

So medical industry price gouging would also impede widespread rapid testing, even if they were available. The odds of something as cheap and easy as a home pregnancy test or ketone strips coming soon seems remote.

By Bonnie LaFleur, Professor of Biostatistics, University of Arizona and Katherine Ellingson, Assistant Professor of Epidemiology and Biostatistics, University of Arizona. Originally published at The Conversation

Since September, the Food and Drug Administration has approved seven COVID-19 tests that yield results in 30 minutes or less, offering hope for vast improvements in test access and efficiency throughout the U.S. Most of these are antigen tests that look for viral proteins and can be processed on portable machines or cards.

The idea behind these rapid tests is to detect symptomatic, pre-symptomatic and asymptomatic infectious people before they can spread the coronavirus. But despite massive distribution of these tests by federal officials – including to date over 40 millionof 150 million rapid tests ordered from the medical company Abbott – COVID-19 transmission has been surging in every state since early November.

This calls into question whether the current influx of rapid tests can actually slow the spread of COVID-19.

In some targeted applications – and if people take other precautions including mask wearing and social distancing – rapid tests can be a valuable tool. But the current state of availability and accuracy of these tests greatly limit how effective they are at slowing the spread of the virus in communities.

Speed Over Accuracy

Rapid antigen tests are an attractive option because in addition to their speed, they are cheap and easy to produce and therefore more broadly available than the more commonly used gold-standard PCR tests in theory. But these attributes come with a trade-off: less diagnostic accuracy. This makes them an excellent candidate for use as a screening tool, though less useful for accurately diagnosing SARS-CoV-2 infection.

One-time testing does not mean that a person can safely travel or mingle without precautions. And while no test is perfectly accurate, there are real questions about the performance of the new rapid tests.

A few test manufacturers reported accuracy between 84.0% and 97.6% in individuals who are tested within five days after developing COVID-19 symptoms. There is, however, an apparent gap between the reported performance of these tests and what is achieved in the real world. Anecdotally, these tests seem to miss recent, mild and asymptomatic infections – in fact, rapid tests are authorized by the U.S. Centers for Disease Control and Prevention only for use in symptomatic COVID-19 patients. And of course, people can still be infected soon after getting tested.

For rapid tests to effectively limit spread of the coronavirus, experts suggest that they must be conducted with high frequency– you might miss some cases, but if everyone were getting tested all the time, you would catch a lot of cases too. But even frequent testing is not a panacea. It’s only one part of an approach that must also include social distancing, mask wearing and other precautions.

A highly publicized example of how a rapid testing strategy can go wrong occurred when President Trump and many in his inner circle contracted COVID-19, likely stemming from a single superspreading event. Everyone was reportedly getting daily rapid tests, but they were largely ignoring other measures like face masks and social distancing. It is likely that someone was infected and asymptomatic, tested negative, and then started the outbreak.

Widespread, Repeat Testing

Detecting pre-symptomatic and asymptomatic individuals who are infectious is critical to controlling the coronavirus. Rapid tests can do this, but only if people are screened repeatedly on a schedule – much as what has been happening in some professional and intercollegiate sports.

The idea is that by testing people early and often – perhaps even as much as every day – rapid tests can catch infected people before they spread the coronavirus to others. But on a national scale, that is a huge number of tests.

Researchers have estimated that the U.S. would need to perform at least 20 million rapid tests per day to drive down infections. The 150 million rapid tests ordered by the government in late August were earmarked for high-risk populations, but would barely cover one week for the population at large. And don’t forget that logistic capabilities, compliance to frequent testing and the infrastructure to act quickly on results all need to happen as well.

A Targeted Approach

There simply aren’t enough rapid tests being produced for the general public to get repeat testing, so the federal government has prioritized deployment of rapid tests to the high-risk population of nursing homes. Federal guidelines for rapid test use in long-term care facilities are a great example of what a testing program might look like – but also illustrate the current challenges in the use of rapid tests.

If even one person in a nursing home tests positive, all staff and residents must be tested every three to seven days until the facility has been free of COVID–19 for 14 days. When a facility has no cases, all staff are required to get tested according to their county’s test-positivity rate – the higher the rate, the more testing is needed.

[The Conversation’s science, health and technology editors pick their favorite stories. Weekly on Wednesdays.]

Yet nursing homes have had problems with accuracy, staffing and costs while using these tests and find themselves again in crisis during the current surge. While the rapid tests certainly helped catch many cases and should be used in these settings, they cannot single-handedly overcome larger issues that contribute to spread in these settings.

Rapid tests can be effective in highly controlled settings where people are tested frequently and other mitigation measures are in place. Look to the success of the NBA bubble as proof. But in other settings where isolation, mask wearing and social distancing are hard to implement or not followed – like nursing homes or the White House – rapid tests have not kept the virus at bay.

Current testing capacity is nowhere near the hundreds of millions of tests per week required to protect the general population. To date, the promise of cheap and convenient COVID-19 tests being the sole means of controlling disease transmission has not been realized.

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  1. ptb

    Level 1. The number of tests required rises enormously because of some frankly idiotic policies, like keeping restaurants open. As if meeting friends for a meal over a small table (which is the purpose of restaurants), facing each other, and eating while seated makes the virus non-contagious. I know this is beating a dead horse, but closing restaurants etc and subsidizing the owners and workers would have been, and probably still is, the cheaper solution.

    Level 2. If you have large parts of the population that don’t care, no amount of testing is going to help.

    A lost cause I’m afraid.

    To make matters worse still, I’m now getting an anecdotal report that industrial supplies for the manufacturing of tests are still lacking and/or have excessive lead times, and they are now being diverted to the manufacturing of vaccines which are higher priority.

  2. Brooklin Bridge

    In the linked UTube, Harvard Professor Michael Mina,MD (associate professor) explains quite a bit about the benefits to the public of inexpensive rapid testing (for transmissibility – not clinical level presence of viral particles). Besides benefits, he touches on projected cost and feasibility of ramping up to scale (20 million per day). Though his open bias is clearly in the “for” camp, I think the video provides good additional information to this post.


  3. SKM

    There seems to be some confusion here. The antigen test will only be useful if very cheap and in the form of a simple absorbant strip and a quick read off like a pregnancy test. Its strength lies precisely in NOT being as sensitive as the PCR which continues to register positive (picking up fragments of viral RNA) long after an individual has ceased transmitting infectious virus. The antigen test only registers positive if the viral load is high and compatible with transmission. This is its strength. The PCR is virtually useless as a tool for infection control for the above reason, as well as its cost, its inaccessibility for most and the long waits for results. The virus undergoes rapid replication and exponential growth starting about 2 days BEFORE symptoms appear rising to a peak soon after this followed by a rapid fall and 5 days later the load for most has dropped below levels necessary for transmission.
    The antigen test if cheap and widely available should/can be used repeatedly. The confusion is in thinking of these tests as being like the PCR i e for use in medical diagnosis as such (especially in hospitals and drs offices) whereas the antigen test is a public health tool aimed at detecting infectious individuals in real time and isolating them immediately. The PCR is clearly useless for this purpose.
    Michael Mina is the go-to person to understand this but even he seems to be having trouble getting the point across to regulatory bodies who insist on worrying about sensitivity as if the antigen test were being proposed for diagnosis in absolute terms.
    We need to know who is actually infectious in real time (the PCR is missing this window at least 70% of the time!!!!) we need cheap, abundant, easy to use tests and we need millions of them, as soon as possible. We urgently need to pick up the pre-symptomatic (and asymptomatic)people, so it makes no sense to only test the symptomatic. This is to misunderstand this totally different use (infection control) proposed for the antigen test

    1. marku52

      Yes, Cheap enough so that anyone can afford to do it in the privacy of their own home. Mina seems to think that 20 million/day is achievable. And this would be a game changer. But the FDA won’t listen to him, because, as you point out, it doesn’t’ meet standards for a hospital. test.

      FDA, WHO, CDC, NIH, all failing to impress. And don’t get me started on Fauci, who finally admits as an aside, that he take 6000IU of VitD and also C. If it’s good enough for him, why isn’t it good enough for the entire US?

  4. Cuibono

    why at this point in time do we not have a comprehensive analysis of the various test performances in both lab and real world settings? Head to head trials of all platforms. Asymptomatic and symptomatic.

    my own experience is that these tests perform pretty well. I am increasingly comfortable using a positive test to make decisions on isolation. I hesitate to use them to tell someone they are not contagious.
    I routinely do PCR on everyone but it takes 24-36 hours.

    1. Adrian D.

      This is a very good question regarding ‘real-world’ comparisons between test methods. However, I’d add that we also need real-world examination of the outcomes of particular parameters involved in the PCR test (particularly the cycle thresholds). I’ve linked to an article in a comment below that highlights some of the issues – or if you want to google it here’s the title – it’s very readable and, I think, very persuasive.

      “Diagnosing COVID-19 infection: the danger of over-reliance on positive test results” by Cohen, Kessel & Milgroom.

  5. Jeremy Grimm

    The PCR test described in the article linked from this post seems a very poor candidate for widespread testing to control the Corona virus now so well and widely spread across the US. I can understand how making the test on cells collected from the nasal cavity might best identify the first signs of an infection … but the delays in obtaining results from the PCR test exceed the time required to prevent the virus from spreading before it begins replicating on a large scale — especially within the “challenged” testing environment characteristic of the US Medical Industrial Complex. I understand neither how the alternative ‘rapid’ tests operate nor how they are applied and did not notice further explanation in the post or its links.

    My first impressions of the PCR test regimen are that it is a most inelegant brute force application of technique that has migrated from research labs to test labs. And I would not gladly submit to having a sample taken from the deepest interiors of my nasal cavity! — whether I paid for it or was very very well-paid to take the test. Besides, given the depth of the US Medical Industrial Complex’s knowledge of effective treatments for the Corona virus distinct from simple treatment of breathing difficulties offers little to no motivation for obtaining a definitive diagnosis of a Corona infection.

    A couple questions come to my mind:
    – How does the Corona virus spread and are there no methods for collecting the medium which accomplishes this spreading?
    If aerosols and droplets and spittle carry the virus as seems to be the case … why are the testers intent on collecting cells from the deepest reaches of my sinuses to support their tests? ?????
    – If aerosols and droplets or spittle are the chief medium for spreading the virus why is this much less invasive medium not collected for test — especially given the delays inherent in the PCR test regimen?
    I thought many efforts have been made to make molecules to attach to various portions of the Corona virus particle, especially its characteristic spikes. Is that not so? If so, I can think of several ways to detect the presence of Corona virus particles. The virus particle + attracted chemical could … fluoresce or some test could test for the concentration of the attracted chemical assuming a wash were effective in removing any molecules of the attracted chemical that weren’t attracted to a virus particle [simple filtering? viruses blocked chemical passed through filter]. There has been some suggestion that not all people infected by the Corona virus are spreaders for the virus … whether symptomatic for Corona or not.

  6. Yves Smith Post author

    I don’t understand your comment. The article makes clear it is advocating antigen tests for rapid testing, not the PCR test, but points out that not enough antigen tests will be available to provide enough testing to meet demand.

    1. Jeremy Grimm

      The article mentions the word antigen once at the beginning of the article. I missed that on my first and second reading before I started my comment — which in retrospect is something of a wacky idea. I did key in on the idea that there is an apparent gap between the reported performance of the rapid tests and their performance in the real world.

      I had an idea I was chasing and infatuated with. I had several months ago largely written off the idea of widespread testing long before reading this post and that strong bias colored and blindered my reading of this post. The gold standard PCR test sounded like a crazy test design and I had the impression the faster cheaper antigen testing was another hollow promise and inaccurate as well.

      Having finally seen that the rapid testing is an antigen test:
      “One, the PCR test, is a molecular test that looks for genetic material inside the virus, and diagnoses a person as “COVID-19 positive.” The turnaround time for results with these tests is usually 24-72 hours. The second, an antigen test, detects certain proteins that are part of the virus, and also diagnoses a person as “COVID-19 positive.” [from “Testing for COVID-19 :: Washington State Department of Health” found in a quick search]
      Now I understand! [although how are the antigens found? it sounds as if they detect a not quite so unique portion of the Corona virus case]

      In a nutshell, my idea:
      Attach a molecular fluorescent marker to an antigen carefully selected to attach with a part of the virus shell unique to the virus. Mix fluorescent antigen with medium to be tested for the virus. Remove the non-attached fluorescent antigen from the test sample [here I am assuming a can-opener — an inexpensive and effective filter]. Examine the filtered virus particles — if any — and marker antigens using a UV lamp.

  7. Adrian D.

    That the PCR test is (still) considered the ‘gold standard’ is worrying. On paper it may be, but it’s performance in the real-world as demonstrated in previous epidemics (SARS-CoV1, MERS, ebola, measles, etc) isn’t particularly golden. Here in the UK we have reports (on BBC & Channel 4) that the Deloitte-run ‘Lighthouse Labs’ have employed practices that were physically dangerous and frankly I wouldn’t trust them to be cross-contamination secure – we haven’t seen any results from ‘External Quality Assessments’ (EQAs) and we’re not even sure that the government is conducting any.

    See this article for a review of PCR performance in previous pandemics – the False Positive rates were considered sufficient a problem in previous epidemics, including SARS-CoV1, for the WHO & CDC to propose specific protocols – but not for SARS-CoV2 for some reason.


  8. M

    Sadly, Covid 19 is not like other diseases, so these various tests often fail to prevent the transmission of infections, e.g., in airplanes or trains or buses or other enclosed spaces. Many infected persons do not have an antibody (antigen) response to Covid 19 infection.

    Thus, antigen tests often fail to detect infections. Many do not have enough virus detected, after it gets into cells, for as long as five or six days after infection, so the PCR tests fail initially apparently during half of the initial, eleven-day period when infected persons are most infectious.

    Thus, I understand how desperately many want to re-start the economy. I sympathize and wish that more government aid was given, as in other countries. However, it is foolish to mislead oneself or others as to the efficacy of the Covid 19 tests. We must accept reality and not live in dreamland.

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