Difference between Antibody Test and PCR Test
Before learning the difference between Antibody and PCR tests let us understand what each of these tests does and how they are performed.
An antibody scientifically known as Immunoglobulin is a "Y" shaped protein that is used by your immune system to identify and fight any foreign element in your blood. Antibodies are proteins that are generated in your body when it fights certain viruses or infections known as Antigens. Antigens are generally defined as a foreign element that triggers an immune response. An antibody test is a blood test that tests for antibodies in your body against a particular substance (here substance means: virus, fungi, bacteria, or parasite).
What is an antigen?
An external substance, which provokes a response from your immune system when comes in contact with the body is called an antigen. Some commonly known Antigen types are viruses, bacteria, fungus, and parasites.
The defense against antigens is provided by antibodies of your immune system. The immune system detects these foreign elements and creates an antibody to fight against them. Each antibody generated is unique and its purpose is to defend the body from a specific antigen.
Antibodies and antigens have a lock and key structure. This means that the antibody developed by your immune system can only fight against a particular antigen. These antibodies are generated by our (WBC) White Blood Cells. These antibodies latch themselves to the antigen in order to destroy them. The production of antibodies in your body continues until all the antigens have been destroyed. This process may take several days to complete.
How is the antibody test done?
In antibody testing as we know that our body is tested for immunity against a certain viral disease that might have affected us before. In this procedure, the blood of a person is taken and is tested for the presence of antibodies that are produced in response to a specific infection. If the antibodies are present in the blood sample, the person is tested positive or considered infected. However, if antibodies are not detected, the person is tested negative or is not infected. For example, if the antibodies, which are produced in COVID-19 infection are found in the blood of a person, he is infected with COVID-19.
Why do we need antibody tests?
Antibody tests let you learn more about your immune system. It tests for the proteins produced by the antibodies that were produced while fighting off the antigen.
PCR stands for Polymerase chain reaction. It is a technique used in the laboratory to make multiple sample copies of a segment of DNA /RNA. Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA) are the two most important molecules of a cell. These acids store genetic information.
A PCR test is performed for the detection of genetic material belonging to a particular virus. In this test RNA of the protein is extracted to get its DNA and thus helping to detect the presence of the virus. After this reverse transcriptase and DNA polymerase are added to the samples as they help in making innumerable copies of the samples. This test shows the trace of the virus if it's present during the time of testing. In this test, a PCR test sample is cloned into multiple samples and test cases are run to detect the presence of the protein produced by the antibody on a PCR machine.
Previously the amplification of DNA implicated cloning of segments which took weeks. As of now, with PCR this is done in Test-Tubes and it takes only a few hours.
This process is done in a laboratory with high pieces of equipment and generally, this process may take up to 48 hours to detect. These tests are very sensitive. They can show false positive results even for microbes that might have been dead already. This means that even if the person might be healthy or might have recovered, the PCR test would still show positive. As there are many cloned samples, all these samples run through different test cases. A special machine called the thermal cycler is used to perform this task.
How is PCR done?
There are three main steps involved in the PCR test. These three steps are repeated for 20-40 cycles. Each step gets executed at a different temperature.
Step 1: Denaturing
At this stage, the DNA is heated to 94-95 degrees Celsius. During this process, the hydrogen bonds between the two DNA strands break resulting in two single strands which act as a template for the production of new DNA strands. While this process goes on it is important to maintain the temperature long enough for the two strands to separate. This process takes up about 15-30 seconds.
Step 2: Annealing
In this stage, the reaction is cooled to around 50-65 degrees Celsius. This step enables the primers to attach themselves to the single strands of DNA. Now the question will arise what is primer. A primer is a single strand of DNA or RNA. These are around 20-30 bases in length. Once the primer is bound to a double-stranded DNA the polymerase enzyme attaches itself to the DNA. This step usually takes about 20-30 seconds. This is when the copying process starts.
In this step, the temperature is increased to 72 degrees Celsius. This process makes a double-stranded DNA due to its temperature tolerance. This step usually depends on the length of DNA and it takes about one minute to copy a thousand DNA bases.
Now you must have realized how the PCR cycler machine works. These three steps are repeated almost 30-40 times. These copies increase in exponential order on the number of times it's repeated.
The main advantage of the PCR test is that it gives you result within few hours.
What does a positive PCR test mean?
A positive PCR test means that the subject is tested positive for a particular virus.
What does a negative PCR test mean?
A negative PCR test means that the subject is tested negative for the virus.
Difference between Antibody test and PCR test
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