RT-PCR vs ANTIBODY test difference for Covid19, key points, specific importance and limitations of these tests with simplified explanation. Both these tests are for different purposes. The Reverse Transcriptase - Polymerase Chain Reaction (RT-PCR) is conclusive as we can know if the patient is infected with covid19 at that particular point of time. The anti-body test on the other hand is non-conclusive and just indicates whether the patient is / was infected with covid19 over a period of time by the report of number and type of antibodies present in the sample. This antibody test is also used to collect data about the number of people who have been infected with this virus in a community where people were in close proximity with the covid19 positive patient and also about their immune system response. DNA was initially thought to be the only medium of genetic information storage and that information was passed only one-way from DNA to RNA to protein. but it was later found that some viruses shift their genetic information from RNA to DNA and these viruses are called retroviruses.


Antibody test data will provide an important insight on the spread of the coronavirus and various factors affecting the collective immunity. It is not possible with RT-PCR.


RT-PCR is the confirmatory test to do if someone has clinical symptoms. The test can also be used to know whether the patient has fully recovered or not.

The antibody test will give data on how many people are exposed to the virus at some point of time, a method to observe and record the extent of virus spread in the community.

-> the antibody based test can be performed in most of the pathology laboratories, while RT-PCR needs special instrument, which many labs may not have.


Suppose a person with clinical symptoms like high fever, dry cough, etc i.e. related to Covid19 symptoms(for the past one or two days) wants to get himself tested. A throat swab and blood sample of this person is taken. 

If the RT-PCR of throat swab shows presence of the virus, it means that he/she is Covid-19 positive. But, in the case of an antibody test, the result of the same patient may be negative, as the antibodies produced by the body to fight against covid19 may take at-least five to eight days time to reach the minimum number of antibodies to be detected. Thus in this case, if only the antibody test is done, the test result of this patient will be negative, even though he is infected with this type of coronavirus.

Let us now look in to another case. A healthy person had mild fever, cough for atleast 14 days i.e. two weeks (sometimes no symptoms at all), and it was not alarming for him to get himself checked. He/She recovered without any treatment or medication and came back to a normal healthy life style. Suppose the health authorities approach this person to test if he is affected by this virus as one of his friend in office was covid19 positive and this healthy person (who had either mild symproms or no symptoms at all) is subjected to both the RT-PCR and the ANTIBODY tests, THE RESULTS OF THE RT-PCT WILL SHOW NEGATIVE (as there is no traces of virus in him and he has recovered) AND THE RESULTS OF THE ANTIBODY TEST WILL SHOW POSITIVE, as there are still antibodies in his/her samples.


RT-PCR is the abbreviation of Reverse Transcriptase - Polymerase Chain Reaction.

firstly let us understand what is PCR (Polymerase Chain Reaction) and then about the RT (Reverse Transcriptase) tests

PCR is a chain reaction, done with a special enzyme named “polymerase”, by which a targeted fragment of DNA (deoxyribonucleic acid) is doubled at every cycle of reaction. 

this is how it works -> 

if there is one copy of target gene in a test tube, in the first cycle of PCR, there will be two, in second cycle four and this will double up every cycle so that there will be finally suffucient number of copies to detect. 

this technology can be used only for DNA as starting material. It cannot be directly used for detection of RNA viruses, like a coronavirus. 

this is where the RT (Reverse Transcriptase) comes in to play and this must be done prior to the PCR test

and that is why, RNA needs to be converted into DNA and this process is called Reverse Transcriptase which is done first, before it is put to the PCR test.

The reverse transcriptase (RT) is an enzyme used to make complementary DNA (cDNA) from a RNA (ribonucleic acid). 

RNA is a high molecular weight compound that functions in cellular protein synthesis and replaces DNA as a carrier of genetic codes in some viruses like the coronavirus. 

RNA has in it ribose nucleotides which are nitrogenous bases appended to a ribose sugar attached by phosphodiester bonds. The nitrogenous bases in RNA are adenine, guanine, cytosine, and uracil, which replaces thymine in DNA.


Understanding the Immunoglobulin Test

this test measures the level of types of antibodies in the blood. The immune system makes antibodies to protect the body from bacteria, viruses, and allergens.

The body makes different antibodies against different invading viruses or bacteria or allergic particles to either neutralize or kill them.

lets look in to the type of antibodies

Immunoglobulin A (IgA) -> This is found in the linings of the respiratory tract and digestive system, as well as in saliva (spit), tears, breast milk etc

Immunoglobulin G (IgG): This is the most common antibody. It's in blood and other body fluids, and protects against bacterial and viral infections. IgG can take time to form after an infection or immunization.

Immunoglobulin M (IgM): Found mainly in blood and lymph fluid, this is the first antibody the body makes when it fights a new infection.

Immunoglobulin E (IgE): Normally found in small amounts in the blood. There may be higher amounts when the body overreacts to allergens or is fighting an infection from a parasite.

Immunoglobulin D (IgD): These are present in very small quantity in the blood.

In response to entry of any ‘foreign invader’ in our body, antibodies are generated by our immune system, which are again specific to that invader. Therefore, if we can detect the presence of specific antibodies in a blood sample, it can be safely presumed that at some point, that individual is exposed to that invader. 

important info - 

1.-> unlike DNA, antibodies cannot be copied in a test tube, and therefore, this test is generally less sensitive than RT-PCR.

2.-> the human body takes some time to generate antibodies after it is exposed to an invader. Antibodies continue to be present in the blood for some time even after the invader is completely eliminated from the body and there are no clinical symptoms.