ELISA tests make it possible to detect with high precision the presence of antibodies in the blood of patients. Besides the detection of the coronavirus, these tests are also used to detect other viruses and organisms; tests for AIDS, toxoplasmosis or measles are also “elisas”.
At the beginning of the pandemic, all the media only commented on PCR tests and rapid antibody tests. Then came the ELISA tests and with them a bunch of doubts that we will try to clear up in the following lines.
1- What does an ELISA test mean, what is it and what does it consist of?
From the acronym Enzyme Linked Immuno Sorbent Assay, the ELISA is a quantitative test that indicates the amount of antibodies, or immunoglobulins, that we have in our blood against a virus. In this case, against the SARS-CoV-2 coronavirus. This laboratory test is also one of the most used to detect patients who have been infected with HIV.
The test only consists of a small blood draw (4 milliliters) and it is not necessary to come on an empty stomach.
2- What are antibodies and which ones are you looking for in this test?
Antibodies, also known as immunoglobulins and abbreviated with the abbreviations “Ig”, are glycoproteins of the gamma globulin type or better said and to understand us, antibodies are defense proteins manufactured by the immune system and are used to identify and combat foreign elements such like viruses, bacteria and toxins.
IgA and IgM appear first, but it is the presence of IgG antibodies against the virus, which appear approximately from day 15 from the start of the infection, which provides probable immunity. Although at present it is unknown for how long, it is usual that over the months or years, depending on the case, the IgG antibodies weaken until they disappear or stop being effective.
3- What is a quantitative antibody test?
In addition to knowing whether or not we have IgA, IgM, and IgG antibodies to SARS-CoV-2, this test will measure the amount of antibodies in the blood sample, also known as the count.
4- What are the sensitivity and specificity of a test?
Sensitivity is the ability of the test to detect disease. It is calculated as the percentage of true positives on the total number of patients. For example, a sensitivity of 98% means that only 2 out of 100 samples have been diagnosed as negative (healthy) when, in fact, they were from infected individuals (positive). That 2% indicates the percentage of so-called false negatives.
Specificity is the ability of the test to detect healthy individuals. It is calculated as the percentage of true negatives on the total of healthy individuals. For example, a sensitivity of 99% means that only 1 in 100 samples have been diagnosed as positive (infected) when, in fact, they were from healthy individuals (negative). That 1% indicates the percentage of so-called false positives.
5- Are the results of the ELISA test reliable?
Very high percentages in these two metrics are mandatory to be able to trust their results. Our laboratory performs the most complete test, which quantifies and measures the 3 types of IgA, IgM and IgG antibodies, using protein S (spike) and N (nucleocapsid) antigens to improve the sensitivity, specificity and robustness of the assay.
6- What is better, a rapid test or an ELISA test?
While the rapid test is a qualitative test where a binary result is obtained, that is, yes or no, as in pregnancy tests; The ELISA test is more complete, requires a laboratory for sample analysis, and provides a quantitative measure of the antibodies present in the sample. The ELISA test is of higher quality, since knowing the amount of antibodies is very relevant: the more antibodies you have in your body, the more likely you are to develop immunity to the disease.
7- When do IgA, IgM and IgG antibodies appear and what do they indicate?
Immunoglobulins A (IgA) are the first to appear, 4-5 days after infection. They are one of the main antibodies in our bodies and are found both in the mucous membranes of the respiratory tract and digestive system and in saliva, in tears and breast milk.
Immunoglobulins M (IgM) are the following antibodies that our body makes to fight an infection, they are detected around 10-15 days and are mainly found in the blood and lymphatic fluid. A positive IgM against SARS-Cov-2 would indicate that we are probably going through the acute or central stage of the infection.
Immunoglobulins G (IgG) are the most common antibodies in our body, they are detectable after 15-20 days and are found in blood and other fluids. They provide protection against bacterial and viral infections and take time to form after infection or after administration of a vaccine. A positive IgG against SARS-Cov-2 would indicate that we have completed the last stage of a viral infection and that we are in the resolution phase.
8- How long does it take to know the results?
ELISA test results will be sent on Fridays from 8 p.m. and will be sent tou you by email.
9- Should I combine an ELISA test with a PCR test?
When a person decides to take a COVID test, especially if they have recently been in contact with someone infected, we advise to get tested with both types of tests, PCR and another one of the serology type, either the ELISA test or the rapid test. The combination of these two types of tests will tell us what phase of the infection we are in: in the first 2 weeks after infection, the PCR test will be positive and the serology negative; later, it will be the other way around: the serologies will be positive and the PCR test negative.
10- How much does it cost and where can I do an ELISA test?
The price of the ELISA test for individuals is 55 euros per person, the price of the PCR test is 125 euros and the price for the rapid antigens test is 50 euros. The tests can be carried out in Madrid, in our clinic at Paseo del General Martínez Campos 46.
Photographs: Quino Al, Tyler Nix, United Nations.
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Since the start of the COVID-19 pandemic, our laboratory has provided its comprehensive clinical analysis and testing services to help tackle the health crisis in Spain. We are working for hospitals, residences and corporate entities and we have contributed to the correct diagnosis of thousands of people and patients.
In our laboratory we carry out PCR tests to detect positive cases with a reliability of 99.9%, together with the rapid antigens tests and the most complete serology tests, the ELISA tests. We also offer the best quick test for the detection of IgM and IgG antibodies against the SARS-CoV-2 virus.