Health : What is the difference between PCR and antibody tests?
What is the difference between so-called PCR and antibody tests to detect the coronavirus? What are the two approaches? The extraordinary circumstances of the epidemic force us to adopt a method on which we have never relied before: that of so-called anti-body tests. This method is intended to be faster, easier and less expensive to implement. According to many, it would detect the coronavirus, just like PCR tests already widely used. But many people believe that the results obtained with a blood sample are more reliable than others.
So why has the PCR test obtained by taking a nasopharyngeal secret been used so far? And what is the difference with so-called antibody tests? Below we take a look at the facts in search of more detailed explanations. We also suggest that you find out which of the two tests to favor according to your situation and whether to take a second test after a certain time.
The test says PCR – what is it?
Let’s start by understanding a little more clearly what the most popular coronavirus tests to date which are today known as the PCR test. And first, what does this abbreviation mean? The name of PCR is the abbreviation of the English expression Polymerase Chain Reaction, polymerase chain reaction in French. Tests of this type are also known as a nucleic acid amplification test. They existed long before their widely known application in coronavirus screening. They are used in particular in screening for HIV, hepatitis B, C and D viruses. They are also used to detect traces of GMOs (genetically modified organisms).
This method is used to search for characteristic genetic material not only in microbiology, but also in oncology, for example, when looking for genetic mutations in DNA and other areas of medicine. It is based on the fact that DNA is a double spiral structure made up of two chains of interconnected substances called nucleotides. There are several types of nucleotides, and they alternate in a different sequence. The nucleotide sequence of one chain matches that of the opposite chain.
PCR tests were the only ones used so far. How do they work and how do they facilitate screening for SARS-CoV-2?
Take, for example, a section of DNA with a sequence known to us. If we split the spiral and get single chains and add its opposite specific chain to the sample and it connects to the one we are looking for, it proves that this given sequence is present in the sample. The connection itself is visualized with a marker, an auxiliary substance that, for example, lights up or changes color during the reaction.
The so-called PCR test makes it possible to reproduce DNA or RNA chains to detect a positive or negative result
We’re talking about DNA, but doesn’t the SARS-CoV-2 virus have RNA? In fact, The method is based on the same principle. This explains why so-called PCR tests are used in screening for SARS-CoV-2.
What does polymerase mean? In this case, we are talking about molecules of very small size. The appearance of a single binding and positive sample cannot be visualized, so when taking the sample it is placed under conditions in order to build a lot of DNA (or RNA) from the strings of the same sequence. In other words, it is a reaction to copy or reproduce these strings. Thus the reaction allows to obtain a positive sample of multiple bonds that we can measure.
In many countries around the world, considerable efforts have been made to limit contact between professionals handling the tests and people who may be positive for the virus. Taking samples in the car is a way to limit this contact
Therefore, with Covid-19, when taking nasopharyngeal secretion, we strive to pick up viruses. The sample is then processed to obtain a sufficient amount of genetic material, which takes time. Then it is checked whether this genetic material belongs to the virus or not. Thus, the sample is said to be positive or negative respectively.
The labs work with nasopharyngeal secretions, because that’s where the virus reproduces. At first, the test may be negative because the virus is not actively multiplying and enough material may not be collected. However, the appearance of symptoms – runny nose, cough, etc. speaks of an actively reproducing virus (multiplication). The sample is then much more likely to be positive.
Antibody test – what is it?
For several months now, a new method of coronavirus testing has appeared. It is meant to be faster and cheaper and it is known as antibody tests or rapid tests. But it was not, at first, assimilated to PCR-type tests. Suddenly, the results of these tests did not enter the statistics of positive people announced by the health authorities. Now that is slowly starting to change. Why ? And what is the method of these tests? Can they really deliver results faster and just as effectively?
In the case of rapid anti-body tests, the method is completely different. The anti body test is also not used only in microbiology.
One way the immune system responds to disease substances is by producing antibodies against them. The antibodies themselves are always specific. In fact, these antibodies are intended for a specific element of the surface of the microbe. They recognize it and contact it and activate other immune responses to its elimination and destruction. That is, antibodies against SARS-CoV-2 will not bind to adenovirus, for example.
The anti-body tests are meant to be much faster and cheaper. But are they as effective?
So, in the case of the rapid or anti-body test, the specialists work with blood samples. They’re looking at the results for traces of anti-bodies that target SARS-CoV-2. Testing positive for these antibodies means that our immune system has encountered the virus and built antibodies against it. The production of antibodies is a time consuming process. There are several types of antibodies, but in this case we are talking about those of the IgG and IgM type. IgM are the arsenal of battle with a new active infection, and IgG carry longer immunity.
IgG and IgM antibodies provide information for two types of immune response. IgM indicates a new infection in progress. IgG indicates longer immunity
At the onset of infection, antibodies have not yet been produced, and even with early symptoms the test is likely to be negative. For this, with the active symptomatology and the negative test, there is a reason for it to be repeated within a few days.
In summary, both types of test have their uses in the fight against the virus.
We can summarize that the PCR test proves the genetic material of the virus and for this to date it has been accepted as the main official method of proof. However, the antibody test helps us prove that we are really sick with Covid-19. It is specific enough to narrow down that it is just this virus. Its implementation is easy, fast and significantly cheaper, and the technology is available to all laboratories.
So when should you take a test and which one should you take?
People exposed to the virus who have been in close contact with a confirmed case should get tested whether or not they are showing symptoms, considers Amira Roess, Ph.D., professor of global health and epidemiology at George University. Mason.
“By identifying people who are positive early in disease progression before they develop symptoms and implementing public health interventions, we can prevent a significant percentage of infections. This is the key, because we have learned that asymptomatic infection is a key factor in this epidemic, ”she said. “Finding asymptomatic individuals will allow us to prevent them from spreading the virus. “
On the other hand, people who have no history of exposure and have no symptoms should not get tested, adds Roess.
The incubation period for the virus is around 5-7 days, but it can be up to 14 days.
If you take the test too soon after exposure, it may be possible that you have a false negative test, notes Dr. Abraar Karan, an intern at Brigham and Women’s Hospital and Harvard Medical School.
A positive polymer chain reaction (PCR) means you have detectable viral RNA. “It doesn’t necessarily say if you’re contagious,” Karan said.
Despite a positive PCR, studies have shown for some people that there is little or no culturable virus after about 9-10 days. This means that a person has had an infection with the virus but tested positive long after they were no longer infectious to others.
However, PCR can remain positive for several weeks after active infection. “It means that you are detecting viral RNA, but not that you are contagious to others,” Karan said.
According to the US Centers for Disease Control and Prevention (CDC), a positive test means you should isolate at home for 10 days. Dr. Sophia Yohe, director of the University of Minnesota Molecular Diagnostic Laboratory and medical director of the Fairview COVID-19 Testing Lab, points out that the test only helps if you want to be active in protecting yourself- same and others.
When to do both types of tests?
At present it is recommended to take the tests for people who have presented symptoms within 9 to 14 days of the onset of symptoms. The antibody test is useful for people with asymptomatic infections who have experienced an illness like covid-19 14 days before. It is also useful if you have been exposed to someone sick with COVID-19 14 days before. In these cases, it can determine if you had the virus in the past.
Experts have not yet determined whether having the antibodies means you have protective immunity against future infections with the virus. In fact, more in-depth studies are needed to determine whether this immunity applies to all people who have been infected. It is also not clear how long this immunity will last depending on the person. But, at present there is evidence showing that such immunity is developing. For now, however, professionals refrain from making generalized conclusions.
Also, it is possible to get a false positive test for antibodies, especially if you had no symptoms of COVID-19. Until we have more data on these key factors, experts do not recommend assuming immunity with a positive antibody test.
Physical distancing and mask use should always continue if you have a positive antibody test.
Do I have to take a second test?
If you have ever had a PCR test and then be exposed or develop symptoms, you should be retested.
In this regard, the health authorities in the United States, for example, advise healthcare workers who have tested positive for PCR to return to work safely if two consecutive samples taken within 24 hours of each other come back negative.
Unfortunately, data is lacking on whether asymptomatic and unexposed people who previously had a negative PCR test should experience periodic testing, according to Yohe.