COVID-19 Diagnostic Testing

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COVID-19 testing involves analyzing samples to assess the current or past presence of SARS-CoV-2. Severe acute respiratory syndrome Coronavirus-2 (SARS-coV-2) is one of the strain/pathogen that causes COVID-19. Covid Testing is basically assessed on their sensitivity and specificity abilities. A test’s sensitivity is the ability of a test to correctly identify those with the presence of disease (true positive rate), whereas test specificity is the ability of test to correctly identify those without the disease (true negative rate).

What are the different types of COVID-19 tests available?

There are two different kinds of tests available for COVID-19.

  1. A viral/diagnostic test that tells you if you have an active coronavirus infection.
  2. An antibody test tells you if you had a previous infection.

Currently there are two types of viral tests– molecular tests, such as RT-PCR tests, that able to detect the presence of specific genetic material from the virus, and antigen tests that detect specific proteins from the virus.

Antibody tests check your blood by looking for antibodies. Antibodies are small proteins made by our immune system in response to infection, such as specific virus. Antibodies can help fight off infection. They can take 1-3 weeks to develop after you have been exposed to SARS-CoV-2 and may stay in your blood for several weeks or more after recovery. Antibody tests should not be recommended to diagnose a current COVID-19 infection, except in instances in which viral testing is delayed. . An antibody test may not show if you have a current COVID-19 infection, because antibodies take several days to few weeks to develop against infection.

RT-PCR versus Antigen Tests

Reverse transcription polymerase chain reaction (RT-PCR) test is highly sensitive and highly specific but it takes several hours to days for laboratory tests; less than an hour for point-of-care tests.

How does RT-PCR works?

A sample is collected from the parts of the body where the COVID-19 virus gathers, such as a person’s nose or throat. Virus has RNA much similar to DNA but with single stranded nucleotide chain. First step is to extract RNA from the virus copy. But RNA is difficult to work in lab than DNA. DNA can be amplified and quantified using a relatively simple, accurate method called polymerase chain reaction or PCR. PCR doesn’t work with RNA, so we convert the RNA into DNA using an enzyme called reverse transcriptase. The resulting complementary DNA (cDNA) developed by reverse transcription is amplified to several times (make billion of copies of it). Primers are added, short fragments of DNA that are complementary to specific parts of the transcribed viral DNA. These fragments attach themselves to target sections of the viral DNA that only the virus has and they give large number of copies of segments so they can be detected and measured. As new copies of the viral DNA sections are built. A probe is added during the PCR process which gives fluorescence whenever a new DNA copy is formed, which is measured by the machine’s computer and presented in real time on the screen.

This test is highly sensitive because it can sense even low levels of viral genetic material in a patient’s sample.

Antigen Test:

This test is done by examining whether viral proteins are present in the throat or nasal fluid. The sample is collected through nasopharyngeal or oropharyngeal swab. This test can detect 4 main coronavirus proteins, spike protein, membrane protein, and envelope protein, and fourth one is nuclear capsid protein in which RNA is embedded. Rapid antigen tests perform best when the person is tested in the early stages of infection with SARS-CoV-2 when viral load is generally highest.

Antigen test is highly specific, but generally less sensitive than RT-PCR test. Antigen tests are relatively inexpensive and can be used at the point-of-care. The currently authorized devices return results in approximately 15 minutes.

Antibody Test:

An antibody test, also known as a serology test, is done with a blood sample that may identify past infection of the virus that causes COVID-19. Antibodies are detected in the blood after infection. They take about 1-3 weeks to develop. If you got lots of antibodies for fighting COVID-19, you probably have COVID-19, and possibly some level of immunity. Running the test is incredibly fast, you can potentially get results within minutes. But this test should not be used for diagnostic procedure of current COVID-19 infection as antibodies take few weeks to develop.

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

Well, it depends upon the test being used. You get the results of antigen and antibody test within less than an hour. The RT-PCR test takes some time from few hours to a day. Currently, RT-PCR is highly sensitive and highly specific test. Antigen test is highly sensitive but less specific as compared to RT-PCR test. Antibody test should not be recommended for diagnosis of current COVID-19 infection as body takes time to develop immunity.