Antigen vs PCR: What’s the Best COVID-19 Testing Method for Schools?

February 17, 2021

Quick Summary:

  • 59% of all COVID-19 infections are transmitted from people who are not showing any symptoms.
  • Antigen Tests gained traction due to the following initial challenges with PCR: high materials and labor cost, demand outstripping lab capacity, and long wait times for results.
  • CDC study at UW shows Antigen tests missed 58.8% of COVID-19 infections among staff and students not showing symptoms (These were False Negatives).
  • State of Nevada pulled Antigen tests from nursing homes after study showed 60% of results were False Positives.
  • Our simulated scenario, if using Antigen Testing, shows the possibility of an additional 9 new infections, for every 10 original infections (due to high False Negatives resulting in further spread).
  • AMDx PanDx™ PCR Testing solves the initial problems with PCR, creating an affordable $20 PCR COVID-19 Test, and delivering Same Day/24 Hr results. 

As schools around the country start to reopen, the COVID-19 mitigation strategies taken by school districts are widely varied. Some schools have elected to not perform routine SARS-CoV-2 testing, some have decided to randomly test a percentage of the population, and yet others have decided to test everyone regularly.

For those schools and universities that have decided to perform some level of SARS-CoV-2 testing, arguably the most important question is:

What type of COVID-19 screening test should we use to test our staff and students?

  • Some of the questions that arise when deciding which diagnostic test to use include:
  • What is the cost?
  • How accurate is it at detecting SARS-CoV-2?
  • How fast will we get results?
  • How easy is this test to administer? Will managing the testing program be a full time job for our staff, or will it be “plug-and-play”?


Are students spreading infections without exhibiting symptoms?

Many schools use “Symptom Screening” methods to help control the spread of COVID-19, but what many school administrators might not know is that most COVID-19 infections are being spread by people that are not exhibiting symptoms. 

COVID-19 infections can be grouped into the following 3 buckets:

  1. Pre-Symptomatic: Signs and symptoms of COVID-19 aren’t seen until up to 14 days after being infected. Symptom screening obviously misses these people.
  2. Asymptomatic: The number of people who carry an infection, yet do not exhibit any symptoms is estimated to be from 17% to 30% of people.
  3. Symptomatic: These are obviously the easiest to find. 

When you combine Pre-Symptomatic and Asymptomatic carriers, a new study published in January 2021 found that 59% of all COVID-19 transmission is from people who are not showing any symptoms.

majority of covid infections are spread asymptomatically

This becomes problematic for two reasons:

  1. People not showing symptoms may not realize they are infected, and thus can’t remove themselves from the population, which further spreads the virus
  2. Test Results from certain types of COVID-19 tests can vary from positive to negative, and negative to positive, based on where a person is in the timeline of the progression of the viral infection.


The Rise of the Rapid Antigen Test for COVID-19

According to the CDC, the molecular RT-PCR test is the “gold-standard” when testing for the presence of the SARS-CoV-2 virus.

When the COVID-19 pandemic rapidly accelerated in the spring of 2020, although PCR tests were, and still are, the most accurate, there were some serious problems with using this type of test to help isolate and stop the spread of COVID-19, namely:

  1. Cost: The PCR test needs to be done in a laboratory, and there were significant costs associated with this test, such as expensive reagents, lab technician staff costs, and overnight shipping to laboratories.
  2. Labs Over-Capacity: Medical testing laboratories use PCR tests for multiple different targets, not just SARS-CoV-2, but at a much smaller scale. Thus when the pandemic hit, labs did not have the staff, high-volume automated equipment, and supplies to perform PCR testing at the scale they needed to.
  3. Wait Times: When labs are over capacity, wait times increase, and in 2020 we saw wait times to get PCR test results be as long as 10 days. This obviously is not helpful when trying to curb the spread of a pandemic.

These three factors led researchers and innovators to turn to Rapid Antigen Testing as a possible solution that would reduce the wait time down to only 15 minutes.

Sounds like an amazing solution, but what trade-offs will schools have to accept if they decide to use Antigen testing?


Is the Rapid Antigen Test Really “Good Enough” for School COVID-19 Testing?

It is commonly known that Antigen tests are not as accurate as PCR tests, but are the “good enough”? Why use a Bazooka (PCR) when you only need a BB gun (Antigen)? 

The first part in understanding the answer to this question is in looking at a direct comparison of the Sensitivity and Specificity of COVID-19 Antigen Vs PCR Tests:

covid-19 antigen vs pcr test comparison chart

Definitions:

Sensitivity: the ability of a test to correctly identify patients with a disease.

Specificity: the ability of a test to correctly identify people without the disease.

True positive: the person has the disease and the test is positive.

True negative: the person does not have the disease and the test is negative.


False positive: the person does not have the disease and the test is positive.

False negative: the person has the disease and the test is negative

The biggest takeaway from this chart:

For this reason, per the CDC, the Antigen Test is not indicated to be used for screening of people that are not showing symptoms.



Elon Musk Tests Both Negative and Positive with Antigen Test?

Elon Musk announced on Twitter that he took the COVID-19 Rapid Antigen Test FOUR times in one day.

The result:

Two results were Positive. Two Results were Negative.

Elon musk tests both positive and negative with the Antigen test


The discrepancy obviously lies in the Sensitivity and Specificity of the test. 

Getting a False Positive result is a waste of resources to unnecessarily quarantine someone. 

Getting a False Negative result, as Elon did, can be dangerous, as the person is unaware they are infected, and continue to spread the infection to their peers.



CDC Study: Antigen Tests Missed 58.8% of COVID-19 Asymptomatic Infections Among Staff and Students  

To remember our definitions, False Negative means: 

I HAVE an infection, but the test results were NEGATIVE.

To prepare for the 2020 Fall Semester, University of Wisconsin System campuses decided to purchase 350,000 of the Rapid Antigen Tests by Quidel, called the Sofia. 

We don’t know the exact reason UW went with the Antigen test, but presumably, it was a factor of quick results (15 minutes), lower cost, and less infrastructure needed.

After UW implemented the Antigen testing program, the CDC collected 1,110 nasal swabs to compare the results of the Antigen test vs results of the PCR test (on the same individuals).


The Results:

The Antigen Test missed 20% of positive infections among those showing symptoms, and missed 58.8% of positive infections among asymptomatic individuals.

Remember, most transmissions are caused by those without symptoms, yet Antigen tests miss 58.8% of those infections.


The Takeaway:

The COVID-19 Antigen Tests currently on the market produce a high rate of False Negatives, the most dangerous kind of testing error.

This means for every 10 infected persons without symptoms, 5-6 of them could continue spreading their infections, albeit unknowingly.




NV Study: Antigen False Positives Rate of 60% Result In Pulling Tests From Nursing Homes

Another memory jogger on definitions:

False Positive means:

I DO NOT have an infection, but the test results were POSITIVE.

In late 2020, Nevada nursing facilities received the Quidel Sofia and Becton Dickinson (BD) Veritor Antigen Tests from the Center for Medicare and Medicaid Services (CMS). The FDA reported these tests as having a high specificity and moderate sensitivity, but this report of accuracy was based on extremely limited data, according to the State of Nevada.


The advertised False Positive Rates for the Antigen Tests in use were:

Quidel Advertised False Positive Rate: 3%

BD Advertised False Positive Rate: 16%

Of the 12 facilities that had performed testing, eight facilities collected specimens for confirmatory RT-PCR testing (on the individuals who tested positive with the Antigen Test), in order to confirm the accuracy of the Antigen Tests.


The Results:

60% were False Positives. (RT-PCR result was negative, when Antigen Test was Positive).


The Takeaway:

The False Positive Rates observed by the State of Nevada were much higher than that which was advertised

Dr. Susan Butler-Wu, Clinical Microbiologist, USC

“[This is] why you can’t take something that’s approved for symptomatic use and apply it to an asymptomatic population,”




Scenario Planner: How Many COVID-19 Infections Could Our School Miss If We Use Antigen Testing?


covid-19 antigen testing scenario


Here is a walkthrough of a possible scenario, based on using the Antigen Tests we have discussed above:

If you have 10 people tested in your school, and those 10 people have COVID-19 infections, we can assume that 50% of those people are not showing symptoms (because they are either pre-symptomatic or asymptomatic).

Of those 5 COVID-19 positive people, we assume that 60% will get a False Negative from the Antigen test. This is based on the data the CDC collected from the Univ. of Wisconsin study.

This scenario results in 3 people infected with the SARS-CoV-2 virus, and yet they think they are negative, so they continue to intermingle with the population.

The next question is:

How many people will they potentially infect with COVID-19, based on the contagiousness of the virus?

According to the Lancet Medical Journal, the R0 of the SARS-CoV-2 virus is estimated to be somewhere between 2 and 3. Other sources predict that it could be as high as 5.7.

Based on this, we are assuming that each infected individual infects an additional 3 people.

In this scenario, this results in an additional 9 new infections, for every 10 original infections.

If you are not testing everyone regularly, or are not testing 100% of the population, an outbreak could be possible.



Implications for Antigen Testing

Based on the studies conducted at University of Wisconsin and Nevada nursing homes, it has been demonstrated that Antigen tests can have a high rate of both False Negatives (60%, UW) and False Positives (60%, Nevada).

Which means one could consider following up BOTH Positive and Negative result Antigen tests with a confirmatory PCR test.

If that’s the case, as Dr. Garner says, why not just start with PCR in the first place?

Dr. Omai Garner, Director of Clinical Microbiology, UCLA Health

"I think what this data says is that if the sensitivity [with the Antigen test] is as low as 40%, potentially you need to be following up with all of your negative tests with a PCR…So now, if your strategy is to follow up both positive and negative testing with the PCR, well, then why don't you just start with a PCR?”


The Solution: AMDx Makes PCR Affordable and Fast for Schools

To get to the solution, we have to remember a couple of things:

  1. PCR is the “gold-standard” for testing
  2. PCR suffered from high costs, demand outstripping capacity, and long turnaround times.

Since we know that PCR is what everyone needs, instead of focusing on the lower accuracy Antigen Test, AMDx has engineering a solution to make PCR testing:

  1. Affordable: At as low as $20 per test
  2. Fast: Same Day Results and 24 Hour Results
  3. Accurate: Our PCR expertise over the last 10 years has been leveraged to keep above industry standard of accuracy

We have been able to do achieve these results by innovating on our 4 key mechanisms of speed and affordability:

amdx pandx innovation fast pcr testing covid
  1. Re-engineered Materials (to lower costs)
  2. High Capacity Automated Lab
  3. Pooled Testing Algorithms
  4. Seamless Software Integration

With these 4 pillars, we are ready to help schools reopen safely.


For more information on how we could help your school, university, or organization, please click here to learn more:

https://amdxinc.com/contact-us/

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