#PCRtest – BioQuakes

As the world faces the Coronavirus pandemic testing has become a hot issue that people are facing in their daily lives. But the question remains, which test is best? It is likely that we will never know the answer to that question, but an understanding of the options available can certainly help the general public to make an informed decision before selecting a method of testing. According to the article from the Financial Times, titled “What coronavirus test does the world need to track the pandemic?” there are two main types of tests for the virus officially named Sars-CoV-2: Antigen tests and Antibody tests. Antigen tests are used to detect whether or not an individual is infected with the coronavirus that causes Covid-19 while antibody tests are used to determine if an individual has been infected in the past.

Antigen tests come in two varieties, a rapid version that delivers results in minutes and another using Polymerase Chain Reaction (PCR) to multiply the genetic material found in a virus containing sample from an infected individual which takes longer. Both the rapid and the PCR tests can be performed on a sample collected using a nasopharyngeal swab but can also be done using a throat swab or saliva sample.

Antigen Testing via Nasopharyngeal Swab

The virus can be detected as early a the first day of symptoms but is most accurate in the first week. This is measured by the cycle threshold where the lower values indicate positive results. No tests are always accurate but the rapid tests have been found to have more false positive and negative results than the PCR tests. In a study performed by Vermont’s Department of Health only four individuals of 65 who had tested positive with the rapid test, also tested positive with the PCR test. Because of this many states require both tests to confirm a positive diagnosis. Unfortunately, the PCR tests have also shown high incidences of false negatives. Regardless of which test is used, the accuracy still seems to be connected to the manufacturer of the test itself. Since the PCR test is thought to be the most accurate because it detects the presence of the genetic material of the virus, it is less likely to provide a false positive. As result, these are best used to identify infection as opposed to letting an individual know that they are not infected.

Antibody tests require a blood sample in order to detect the presence of antibodies against Sars-CoV-2 because once an individual is infected with the virus their immune response creates IgM antibodies that are present close to the time of infection and IgG antibodies that are produced longer after infection. These antibodies are proteins produced by the white blood cells that help to fight the virus, so if they are present in an antibody test, is clear that the individual has already been infected. When an individual is infected with a virus, lymphocytes called B cells are triggered by the presence of the antigen of that virus, and bind to the antigen using an antigen receptor. This then causes the production of cells that produce proteins called antibodies which have the same shape and structure as the antigen receptors of the B cells making them helpful in directly defending against antigens present in body fluids. Antibody tests provide a quick result and are preferred because they help people to determine if a person can return safely to society with less risk of infection.

As the pandemic became a global issue many tests began to be developed but the antigen tests were more commonly performed in the public while the antibody tests were reserved for private companies due to the need for more materials like swabs and reagents to perform the tests. Both tests are being created all over the world but some countries are producing more than others and the US was slow to get started but has increased production more recently. Because the tests must be of good quality, and the staff that are using them must be trained well, it is hard to produce them fast enough.

A study published by University of Colorado Boulder and Harvard University researchers, states that rapid tests could help the world come close to eliminating COVID-19. The study focused on whether sensitivity of the tests, or turnaround times of getting the results is more important. By using mathematical formulas, different scenarios and three locations (a 10,000 person population, a university setting and in a large city) they came to conclude that when trying to slow the spread, frequency of testing and turnaround time is more important than the sensitivity of the test.

It is important to note the difference between the PCR test and the rapid antigen test. A PCR test, which uses polymerase chain reaction technology (hence the name) to detect traces of the virus’ genetic material. A rapid antigen test does not trace the genetic material but instead looks for specific proteins on the surface of the virus, known as antigens. A PCR test can detect one SARS-CoV-2 RNA molecule for a positive test result, the rapid antigen test needs thousands of virus particles for a positive test result. An antigen is present on the outside of a pathogen. In adaptive response to pathogens, dendritic cells place these antigens on display. The antigens are what allows the T-helper cells to recognize the antigen and trigger the cell mediated and humoral response. Therefore, if your body is fighting against COVID-19, the antigens would be displayed on cells and the rapid test may recognize them.

SARS-CoV-2, the virus that causes COVID-19, antigens shown on pathogens surface

In one of the scenarios a large city had widespread rapid testing two times a week and they reduced the infections by 80% compared to widespread PCR testing done two times a week that only reduced infection by 58%. This scenario shows that because two thirds of infected people do not show symptoms as they wait for their results they are not quarantining. If people receive the positive tests results sooner, they can self-isolate sooner.

People have felt hesitant about rapid testing since it is less accurate than PCR testing and may miss cases where levels of infected particles are too low. However, it has been shown that there is a short time period where PCR testing will show a positive, and rapid tests won’t. This is because infected particles can go from 5,000 to 1 million in less than 24 hours. During this short window of time it is also likely that the patient is not contagious yet.

COVID-19 Rapid Antigen Test

Personally, I think that having more accessible rapid tests will be a huge help to curbing the virus. Even with less sensitivity, if a person tests positive then they self-isolate up to up to 48 hours quicker, if tests results take longer, then even more. This means that less people will get infected. I know that when family members have gotten tested, especially in the beginning of the pandemic, tests took 5 days when the labs were backed up. If they had not been properly quarantined during the 5 days, this could pose a danger to others. Therefore, I think that the accuracy can be put aside for the speed of the test results.

So, what do you think? Is rapid testing the inexpensive, fast, key piece to curbing the infection rate, or is the accuracy of tests more important?

BioQuakes

AP Biology class blog for discussing current research in Biology

Tag: #PCRtest

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