Coronavirus Antigen Test: Pros and Cons

It is estimated that approximately 400,000 people per day can receive one in the United States after the nationwide rollout of diagnostic tests for active coronavirus infections is slow.

However, some public health professionals need to test many of the US population of 330 million daily for people to return to work or school safely, new before they get out of control. It states that it is necessary to identify the outbreak. Others suggest that checking about 900,000 people a day is enough.

In any case, almost all current tests used to diagnose infectious diseases work by identifying the genetic material of the virus. This is a technology that is difficult to extend further.

Deborah Birx, White House's coronavirus coordinator and others have touted another option: antigen tests, Antigen testing detects the presence of viral proteins in biological samples such as saliva and tissue swabs from the nasal passages. Antigen testing is usually cheap, results are obtained within minutes, and like genetic testing, it indicates a vigorous infection.

While some scientists are optimistic that antigen-based methods solve testing problems, others remain skeptical, and such tests are far more than nucleic acid tests. He points out that it is inaccurate and may not be as easy to scale as its proponents claim. Everyone wants a test to be cheap, accurate, and fast, but in fact you can only ever have two of those options.

Developing an antigen test is "not that simple. Rather than performing all analytical procedures in expensive, specialized equipment in the laboratory or clinic, as in the case of testing for viral DNA or RNA, antigen testing involves most, if not all, of these procedures. Incorporate into a paper-like strip. Similar to a pregnancy test, returns a simple "yes" or "no" answer.

It's a lab on a swab. With most setups, a sample of bodily fluid is collected using a nasal swab or related procedure, then mixed with a few milliliters of a liquid, typically a sterile buffer solution. A few drops are spotted on one end of a test strip. Capillary forces pull the liquid over copies of two different antibodies specific for the same viral protein. If both antibodies spot their target—a positive test—the strip generates a signal, often a color change. This signal is generally read out by a person visually, although some setups use small readers to improve the accuracy.

There are many things that trigger a signal. In some tests, antibody binding triggers a chemical reaction or exposes a fluorescent marker attached to the antibody.

Another test that challenges FDA approval is that an electrical readout after antibodies on an electrochemical sensor bind to their target antigen.

The challenge is

  • finding the right antibodies. Both antibodies must bind to a single viral protein, such as the spike protein SARS-CoV-2 uses to enter cells, but at separate sites. They have to find two antibodies that don't interfere with each other.

  • Another challenge is weak signals. Genetic tests use the polymerase chain reaction (PCR) to amplify tagged DNA or RNA sequences, making it easy to reliably identify just a few copies of a virus. That gives PCR tests for the SARS-CoV-2 virus about a 98% sensitivity and near perfect selectivity, meaning almost every active infection is detected and only in very rare cases does someone uninfected receive a positive test.

  • Antigen tests don't amplify their protein signal, so they are inherently less sensitive. To make matters worse, that signal gets diluted when samples are mixed with the liquid needed to enable the material to flow across test strips. As a result, the sensitivity of most antigen tests is between 50% and 90%. This means that one in two infected people can be mistakenly told that they are not infected with the virus.

However, there are also advantages to antigen testing.

  • It does not require the expensive equipment or chemicals needed to perform the PCR, making it easier to use as a point-of-care test in clinics, emergency centers, hospitals, and even businesses and schools.

  • It also has lower execution costs because it does not require trained professionals. There are several point-of-care PCR tests, but in most cases you will need to send the sample to the lab for manual processing.

  • And the fast results from an antigen test mean that people who test positive can be isolated quickly, before they risk infecting others.

  • Another advantage is scalability. Once researchers settle on effective antibodies, the tests are easy to manufacture in bulk, and running them doesn't require additional reagents as PCR tests do.