Antigen Testing

What is a rapid antigen test? 

A rapid antigen test detects the presence of acute infection. The type of test is a lateral flow immunoassay and can quickly identify an infection (within 30 minutes). It is best at detecting when someone is sick or has a high viral load. It is similar to other rapid tests that we perform at the Health Center. 

How does a rapid antigen test compare to the PCR test through Broad?

PCR testing is the “gold standard.” It is a more accurate test with higher sensitivity and specificity than rapid antigen testing. The disadvantage is that the test takes longer to result. 

What is the sensitivity and specificity of rapid antigen testing? 

The sensitivity (probability the test correctly identifies an individual infected with Covid-19 – positive result) is variable but ranges from 75-80 percent, with sensitivities upwards of 97.5 percent. The variance in this value is impacted by a number of variables depending on current viral load, sample collection, and test processing. The specificity (probability the test correctly identifies an individual without Covid-19 – negative result) is considered to be at least 99.6 percent.

What does a positive result mean? 

This is a non-diagnostic test. If you have a positive result, we will run a repeat antigen test to confirm. Additionally, all antigen tests give us an idea that you may be infected with Covid-19, but ultimately the test of record is our diagnostic PCR test. If you have two positive antigen tests, you will need to isolate until your PCR test results. If you have a positive PCR test, you are diagnosed as having a Covid-19 infection.

Why do we run the antigen test again if the first test resulted positive? 

By running a second test, two positive test results indicate a higher likelihood of Covid-19 infection. If the first test is positive but the second test results negative, it is less likely that you have a Covid-19 infection. You will not be required to isolate as we await your PCR test result. 

What does a negative result mean? 

It suggests that you do not have Covid-19 infection or you may have a Covid-19 infection with a low viral load. This is why you can have a negative antigen test but a positive Broad PCR test.

What is viral load? Why does it matter? 

The viral load is how much virus exists in the body (and in the nose). PCR does a very good job at finding a very low number of viral particles. Antigen testing is less good at finding very low levels of virus and better at picking up higher levels of virus, which indicates a more severe infection.

Why did we start using rapid antigen testing?

If it is not a diagnostic test, what is the point of using this test? It is a very useful point-of-care screening tool that will help quickly identify if someone may have Covid-19. More importantly, if someone is positive on the rapid antigen test, this may be indicative of a higher viral load and an infection that is more likely to be transmissible. If an infection is not detected on antigen testing but is detected with PCR testing, this can indicate a lower viral load and subsequently a lower risk of transmissibility. All antigen testing will be confirmed with a PCR test.

What is the utility of serial testing, or testing every day or every few days?

As we do with PCR testing, serial testing increases the overall specificity of a test. Though the sensitivity of a rapid antigen test is lower than PCR, a rapid antigen completed test daily or every few days would increase the overall “net” sensitivity (probability the test correctly identifies an individual infected with Covid-19 – positive result) of the rapid antigen test. 

  • Here is a summary of the key differences between the two tests 
Broad Institute PCR
Antigen Test
Also known as…
Diagnostic test, molecular test, RT-PCR test
Screening Test
How the sample is taken…
Nasal
Nasal
How long it takes to get results…
24 hours following receipt by the Broad Institute (Boston).
30 minutes
Is another test needed…
This test is typically highly accurate and usually does not need to be repeated.
Positive test results will be confirmed with a Broad PCR test.
  • As noted above, the PCR test result will be recognized as the test of record. The following matrix outlines what result will be recorded depending on the outcome of each test.
PCR Test Positive
PCR Test Negative
Antigen Test Positive
Positive for Covid-19 infection
Negative for Covid-19 infection
Antigen Test Negative
Positive for Covid-19 infection
Negative for Covid-19 infection