Explained: How antibody test for COVID-19 is different from PCR test

Viral infections are mainly identified by two kinds of tests– genetic and serological. Genetic tests can identify infections that are active, but cannot be used to detect past infections. To trace how infections like the novel coronavirus have spread so far, it is important to detect people who contracted the disease in the past and have recovered. This is what serological tests seek to determine.

How are the two different?

The genetic test is conducted on a swab collected from the back of the throat, a liquid sample from the lower respiratory tract, or a simple saliva sample. For SARS-COV-2, the virus’s RNA is first converted into DNA. By a process called polymerase chain reaction (PCR), DNA fragments in the sample are copied exponentially — one is copied into two, the two are copied into four, and so on.

Unlike genetic tests, which look for RNA in swab samples, serological tests work on antibodies in blood samples. Hence, they are also called ‘antibody tests’. Antibodies, or protective proteins produced by the immune system to neutralise pathogens such as bacteria and viruses, are present in one’s bloodstream for a considerable period of time after the infection has gone.

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To disable a pathogen, the antibody latches to a unique protein molecule on pathogen’s surface, called an antigen. Serological tests use antigen molecules to detect the presence of antibodies relevant to the infection. Generally, a blood sample is placed in a test tube that is lined with antigens on the inside. If the relevant antibodies are present, they latch on to the antigens.

Such tests are relatively inexpensive, and can display results within a few minutes.

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