Antigen testing: what does this mean in the fight against COVID-19?

“Antigen tests can be done immediately when the specimen is collected. These are point-of-care tests where the specimen is collected from the patient. A minimally trained individual can run this simple test and the test results are available in about 15 minutes. The patient is able to get the test results before they leave the site. The advantages of the antigen test are that it's rapid, it's relatively inexpensive, it's very easy to perform and we have the results that can be used to manage the individual patients.”

Dr Patrick Murray, the Vice President of Medical Affairs Microbiology at BD (Becton, Dickinson and Company) provides clear insight into the benefits of an antigen test, how it works compared to a more sophisticated but lengthy PCR test and the relevance of using antigen tests to rapidly test and allow for retesting for COVID-19 within the current phase of the pandemic.

Can you explain in simple terms what is the COVID-19 antigen test?

The COVID-19 antigen test is directed at the “body” of the virus. We should think of the virus as having two parts. One is its genetic information or RNA, and the other is the body that surrounds that genetic information, and that body is made of proteins or antigens. The antigen test is able to detect the presence of those virus proteins in a patient's sample, such as a nose swab. So, the antigen test is directed at the proteins of the virus and the molecular tests are directed at the virus genetic information.

Why would you use an antigen test over a PCR test?

There are important differences between PCR and antigen tests. I will start off by describing the PCR or the molecular test. These are very sophisticated tests that need to be performed in a clinical laboratory by highly trained technologists. For the PCR test, the specimen is collected from the patient and is sent to the clinical laboratory where the testing is performed and then the results are sent back to the physician who is taking care of the patient. That process can take a number of hours or even days, depending on the volume of work that is being performed by the clinical laboratory. So, in many sites, the PCR test results are not readily available.

In contrast, the antigen test can be done immediately when the specimen is collected. These are point-of-care tests where the specimen is collected from the patient, a minimally trained individual can run this simple test and the test results are available in about 15 minutes. The patient is able to get the test results before they leave the site where the specimen was initially collected and the testing performed. The advantages of the antigen test are that it's rapid, it's relatively inexpensive, it's very easy to perform and we have the results that can be used to manage the individual patients.

What is the accuracy of an antigen test?

When the test was introduced for this pandemic, we saw that this test was not as sensitive as what we see with the molecular (PCR) test. The molecular tests are designed to take a single piece of genetic information and amplify that or make many copies before the test is actually performed. Hence, with a molecular test, you literally can have a million copies of what you’re trying to detect.

In contrast, with the antigen test there's no amplification of that target. In other words, you measure only what's already there in the virus. In this case, there's about a thousand copies of the antigen that we're trying to measure, and you would expect that it's not going to be as sensitive as measuring a million copies of the genetic information in the molecular test.

On the surface, it may seem like a less sensitive test is not good, but what we've learned, particularly with this coronavirus infection, is that early in the course of a patient’s disease you see the person is shedding virus that can cause infections. As you move through the life of the disease in the second, third, fourth week, you still have a positive molecular test, but it’s detecting versions or parts of that virus that are not infectious anymore.

What we want to be able to do with a diagnostic test is to check whether a patient has an infection -  and both the PCR and antigen tests can do that. Secondly, we look if the patient can spread that infection to other individuals and in this particular case, the antigen test turns out to be a better test because it gives a positive result during the time when the patient is shedding the virus that's infectious for other individuals. The difference between the antigen test and the molecular test is that the antigen test, although it's less sensitive, it is particularly sensitive during the time when the patient is shedding infectious virus.

Can you explain the relevance of the test in Africa, including Kenya, Ghana and South Africa?

One of the challenges – and this is really true in many countries – is that you need to have a diagnostic test that will give you the results rapidly. The challenge with the molecular test is that it has to be run by a sophisticated laboratory with sophisticated technologists. So, there can be significant delays in results and the test won’t have much value because you can’t use it for rapid diagnosis. The advantage of the antigen test is its speedy results, the simplicity of the test and the ability to get those results and manage the patients very effectively.

Do both the PCR and the antigen test have a role within the same healthcare setting?

It is very important to understand that each test has a role in diagnosis. These tests measure two different things, and they do it in very different ways. The PCR test measures the genetic information or the RNA that is in the virus and the antigen test measures for the proteins that form the shell of the virus.

If you're screening a population for the presence of this virus, it's useful to be able to do the initial screening tests very rapidly – and that means using the antigen test. We know that the antigen test is not quite as sensitive as the PCR test, and so for a population of patients, let's say that they have symptoms of the disease, the antigen test enables rapid diagnosis. But if that test is negative, then the patients can be retested with the PCR test to confirm that they are truly negative or that they have low levels of the virus that can only be detected by the PCR. The tests work together, in parallel.

In conclusion

I think it's very important that as we look at antigen tests, we have to remember that there's a number of tests that have been developed and that not all tests are really the same or equal. There are some tests that work better than others, and so I think for any country or region that's looking to adopt antigen tests, it's very important to look at their performance. It is important to look critically at the available tests and select the best tests to satisfy the needs of the region that will use them.