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    SARS-CoV-2 Serological Tests


    What is a serological test and how is it done?  Serological tests have become an essential tool in the control of the pandemic we are facing worldwide, and consequently the demand for this type of technology is constantly increasing. We can identify two general groups of tests: on one hand there are Diagnostic test, which allow us to identify if a patient is infected with coronavirus at the time the test is performed, and on the other hand, there are serological tests, which are those that detect antibodies and therefore, they give information on whether an individual has been exposed to SARS-CoV-2 and if their immune system has reacted against this virus. 

    PCR Tests are not serological SARS-Cov-2 tests

    There are numerous ways of detecting SARS-CoV-2 in the different stages of infection depending on the type of viral or immunological information the test identifies and depending on the type of technology we use to detect this information:

    What is a PCR?

    Molecular tests are diagnostic tests, and they detect the active coronavirus infection by identifying the virus’s genetic material. This kind of tests are usually highly accurate and specific, and it is the most reliable way of detecting the infection. In this group we place PCR, which is a method of diagnosing infection in nasal by taking throat swab or saliva samples.

    What is an Antigen Test?

    Antigen tests are also diagnostic tests, because they identify the active coronavirus infection, but in this case, they do it by detecting specific proteins from the virus.  Antigen tests are more likely to miss an active COVID-19 infection compared to molecular tests.

    What is a Serological Test?

    Antibody test are also called serological or serology tests and they show if there has been an exposition to coronavirus in the past.  They do not diagnose active coronavirus infection at the time. One of the most common uses for this test is to study the seroprevalence of SARS-CoV-2, in order to evaluate whether the majority of the population have reacted to the virus  and register data about their immunity to the virus, which could be crucial in the future for the vaccines efficacy analysis. 

    Different Types of SARS-COV-2 testing methods 

    How PCR Tests work?

    PCR is a method to rapidly make millions to billions of copies of a specific DNA sample, allowing to take a very small sample of DNA and amplify it to a large enough amount to study in detail, in this case, in order to find in that sample COVID-19  genetic material.

    Is ELISA a Serological test?

    There are different methods to detect both antibodies and antigens (viral infection). One of the tests that are used for this purpose is the Enzyme-linked Immunosorbent assay (called ELISA) which is a commonly used laboratory test to detect antibodies or antigens in different sample types (serum, plasma, saliva,… among others). 

    Therefore, ELISA Test can be a serological test, in fact this format will allow us to access to quantitative and qualitative information about the response of the patient to SARS-CoV-2 with high reliability and sensitivity. However, Serological test can be also preformed by other methods, as rapid antibody test, which sensitivity and reliability is lower, but they are also easier and faster to perform. 

    What is a rapid COVID-19 test and how accurate it is? 

    Rapid tests are also able to detect either antibodies or antigens (viral infection), but they detect it by a different technology. Even though positive results are usually highly accurate in this kind of tests, negative results may need to be confirmed with other procedures (as PCR or ELISA Tests) which means that they are less reliable.  On the other hand, rapid tests are faster and easier to perform, and it is efficient when detecting positive viral infections.


    How is a positive result detected in a serological test by ELISA? 

    The enzyme-linked Immunosorbent assay (ELISA) technique is an immunological assay based on the detection of proteins, antibodies, peptides, and hormones. In an ELISA, an antigen is immobilized to a surface and then complexed with an antibody that is linked to an enzyme.  Detection is accomplished by assessing the conjugated enzyme activity via incubation with a substrate. Then it will appear an observable color to measure the result using a spectrophotometer or a colorimeter. In this sense, this type of test is for professional use only, due to the need of appropriate resources and facilities to perform it.

    How long after exposure is COVID-19 detectable?

    PCR Tests detect viral nucleic acids directly, and therefore can identify recently infected donors who have not yet produced antibodies. On the other hand, Serological tests can detect different classes of antibodies (immunoglobulins) in different moments after the infection: IgM (immunoglobulin M) is the first to be generated after the infection, indicating that the individual is initiating a response to the disease. IgG (immunoglobulin G) is produced at a more advanced stage of infection and can persist over time, informing, until months later, that an individual has suffered from the disease; and immunoglobulin A (IgA), which is produced in early stages, and can also be detected in late stages. Median seroconversion time is 4 to 6 days for IgA and IgM, while is 5 to 10 days for IgG from onset of symptoms. However, our data suggest that IgA detection may improve the diagnostic result in the early stages of infection. (Contact us for more information about the assay).


    Source: Compilation based on data from internal validation (Immunostep). 


    Are serological tests safe?

    Can two different tests give a different result?

    Two different tests can give a different results for two principal reasons: firstly, the type of test that you are using is not the most appropriate due to the stage of the viral infection you are facing, which is the reason why usually the different types of test are complementary to each other; and secondly, the reliability of the specific test (usually method) you are performing is not enough. For a test to be reliable, several variables are analyzed, the most important of which are the sensitivity and specificity of the test. 

    • The sensitivity of a test is the ability of the test to detect the disease, which is the probability that for a sick subject a positive result will be obtained in the test. 
    • Specificity, on the other hand is the probability of correctly identify a healthy individual, which means, the probability that a negative result will be obtained for a healthy subject.

    Both variables are essential for a test, since a high sensitivity avoids false negatives COVID-19 results, while a high specificity avoids false positive COVID-19 results, for example, because the test detect antibodies against the influenza virus or another type of coronavirus different to the SARS-CoV-2.

    Are all tests equally effective?

    Carlos III Health Institute carried out a reliability study of various rapid diagnostic techniques for COVID-19, according to this study the antibody tests available all over the world by then proved a sensitivity of 64% when applied to patients without taking into account the time of evolution of the disease, and 80% in patients with more than 7 days of evolution. 

    Additionally, four research groups of the National Centre for Biotechnolgy of the Spanish National Research Council. (CNB-CSIC) worked during the lockdown to improve this percentage of reliability in serological tests and their investigations resulted in the identification of antibodies reaction against a new non-structural SARS-Cov-2 virus protein (protease) which allowed the creation of a new serological test with a reliability of 98%. This is the test that we manufacture at Immunostep, in our laboratories in Salamanca, in ELISA kit format, one of the most reliable proven technologies for serological analysis of the population, and based on procedures that are frequently performed in hospitals and specialized laboratories.