Covid-19 Testing: Performance and results

There are numerous ways of detecting SARS-CoV-2 infection and antibodies, PCR are diagnostic tests that identify COVID-19 genetic material, and 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.  Antibody test, on the other side, also called serological or serology tests, show if there has been an exposition to coronavirus in the past.

▌ How COVID-19 tests work?

There are different methods to detect both antibodies and antigens (viral infection).  The previously named 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 order to find in that sample the genetic material of the virus.

The Enzyme-linked Immunosorbent assay (called ELISA) is a commonly used laboratory test detect antibodies or antigens in different sample types (serum, plasma, saliva, among others).  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 and the result is measured using a spectrophotometer or a colorimeter.

On the other hand, rapid tests are also able to detect either antibodies or antigens (viral infection), but they detect it by a different technology that usually works like a pregnancy test.  They are faster and potentially cheaper but they are also less accurated than laboratory methods. 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.

▌ How long does it take to get COVID-19 test results?

Currently there are many different tests and ways of reading COVID-19 test based on the method is performed. Most tests to diagnose COVID-19 require a swab of the nose, or the part of the throat behind the nose, by a health care provider.

Some tests use saliva or other types of collection methods. The swab or sample must be sent to a laboratory for analysis in the most reliable tests, however some tests allow the patient to collect the sample at home and then send it to a lab for analysis and there are also authorized one at-home tests that allow individuals to collect their sample and run the test completely at home without needing to send anything to a lab.

An ELISA, which is a test that identifies antibodies, is performed in a laboratory and the procedure lasts round 2 hours. Antigen rapid tests, on the other hand, give results in less than 30 minutes, they are not processed in laboratories and are cheaper to produce. Finally, the PCR test needs also to be performed in laboratory and the procedure takes around 4 hours.

▌ Is a COVID-19 test safe?

The have been several fake news about the possibility of damages when performing some of the COVID-19 tests, however, all the tests we have talked about before are completely safe. Some of the side effects that have been attributed to these assays are brain damage, or even SARS-CoV-2 infection.

In order to perform a PCR, a sample from the nasal passage is taken to the patient, that technique have never caused, and it is not possible to cause any brain damage or any other complication related with it. Furthermore, testing kits are not able to infect any patient with the virus they are trying to identify.

▌ In which cases a false positive can be given in a COVID-19 test?

One of the most common quarions about serology testing is  how do I know if a test is effective in detecting covid? In this sense is important to be carefull and find out about the sensitivity of the tests, which should be as close to 100% as possible.

Furthermore, there are two main reasons a test can give a false result: 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. When a test is not very specific, it could detect another proteins or virus from another coronavirus that is not sars-cov-2, and this could produce false positive tests.

▌ How long do you have to have the COVID-19 to detect it?

PCR Tests detect viral nucleic acids directly, and therefore they can identify recently infected donors who have not yet produced antibodies, but it can take almost a week after exposure to COVID-19 to register a positive test result.

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.