Why we offer both serologic and PCR testing

By having the option of using the antibody and/or molecular types of tests, the treating physician can broadly assess where a patient is in the disease process. Those with positive PCR tests and/or positive antibody tests can be isolated from others. They will be advised to notify their contacts so that they can also be quarantined.

When performed on large numbers of people, PCR and Antibody Testing can give a snapshot about community spread and increase our knowledge about the pandemic.

Serology Antibody Assessment

The serologic antibody test has been designed to be specific to antibodies developed to the SARS-CoV-2 virus. It helps determine whether the patient has been exposed to the virus, and may be indicative they are developing immunity to the virus.

Antibodies against SARS-CoV 2 virus are detectable early in the course of infection, usually about 7-10 days after symptoms begin. Some patients infected with SARS-CoV-2 virus never develop symptoms, but still make antibodies. These people are called “asymptomatic carriers”. Antibody tests can be useful in understanding how many people have been infected or exposed, and how far the pandemic has progressed.

RT-PCR Testing

We offer a molecular test to diagnose COVID-19 infection. The molecular test is called a “Reverse Transcriptase Polymerase Chain Reaction Test (RT-PCR test)”. PCR tests are used to directly detect the presence of viral RNA in the nasopharyngeal sample. By detecting viral RNA, which will be present in the body before antibodies form or symptoms of the disease are present, the tests can tell whether or not someone has the infection very early on.

The value

Testing for the presence of IgM and IgG antibodies is meant to complement PCR testing in the diagnosis and management of SARS-CoV-2 infections. Both tools are needed for a comprehensive response to infection control .

How Should We Interpret IgM/IgG Serological Test Results?

The present IgM/IgG serological assay is designed to complement RT-qPCR in the diagnosis of SARS-CoV-2 infections. Table 1 shows the clinical interpretation of all possible scenarios that can be encountered when testing a patient with both RT-qPCR and an IgM/IgG serological test.

Table 1: Variation of the Levels of SARS-CoV-2 RNA and Antigen, IgM and IgG after infectionTable 1

This table is based on the current knowledge about the rise and fall of SARS-CoV-2 RNA and antigens, IgM antibody and IgG antibody (Figure 1) and the correlation of these level variations with the initial time of infection, onset of symptoms and recovery phase3-5. As shown in Figure 1, serological tests are recommended to be used on patients at least 3 days after onset of symptoms or 7-10 days after infection with the virus3-5.

The key takeaway is that the results of RT-qPCR and IgM/IgG serological tests do not necessarily need to agree. A disagreement between the two tests, if any, can often be traced to the after-infection time points at which the tests were performed. Overall, while RT-qPCR testing may be appropriate for the detection of the SARS-CoV-2 virus during the acute phase, IgM/IgG is an appropriate test during the chronic phase. Since the exact time of infection is often unknown, combining RT-qPCR and IgM/IgG testing can improve the accuracy of the COVID-19 diagnosis.

Figure 1: Variation of the Levels of SARS-CoV-2 RNA and Antigen, IgM and IgG after infection