Interpretation,recommendations and reliability of Serological tests in sars-cov-2?

The acquired immune system of our body produces antibodies against the offending virus like sars-cov-2. These antibodies can be used for diagnosis of active disease as well to identify the immune individual.

 

Unlike a test designed to diagnose an active COVID-19 infection (specifically from the SARS-CoV-2 virus), serological tests can help identify individuals who have developed an immune response to the virus, either as part of an active infection or a prior infection. 

 The tests detect the presence of antibodies in the blood – if antibodies are present, that indicates that the person has been exposed to the virus and developed antibodies against it, which may mean that person has at least some immunity to the coronavirus. In the early days of an infection when the body’s immune response is still building, antibodies may not be detected, which is why serological tests should not be used as the sole basis to diagnose or exclude infection with the SARS-CoV-2 virus.

 The levels of antibodies begin to increase from the second week of symptom onset. Although IgM and IgG ELISA have been found to be positive even as early as the fourth day after symptom onset, higher levels occur in the second and third week of illness.

Some of the studies have shown that IgM persists till 4th week of illness and beyond. The delayed disappearance of IgM usually occurs in patients with higher viral load.

The selection of the appropriate timeframe is essential for the detection of immunity.

ELISA-based IgM and IgG antibody tests have greater than 95% specificity for diagnosis of COVID-19. Majority of antibodies are produced against the most abundant protein of the virus, which is the nucleocapsid. Therefore, tests that detect antibodies to NC would be the most sensitive. The host attachment protein, receptor-binding domain of S (RBD-S) protein and antibodies to RBD-S would be more specific and are expected to be neutralizing. Therefore, using one or both antigens for detecting IgG and IgM would result in high sensitivity. Antibodies may, however, have cross-reactivity with SARS-CoV and possibly other coronaviruses. IgM and IgG antibodies to these sites can be detected by a number of methods including ELISA and immunochromatographic testing. 

 What is the advantage over RT-PCR?

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 PCR becomes negative as the virus is lost from an individual. A negative PCR does not indicate if the person has been infected and may be immune. Antibody testing may allow mass testing of the population and identification of individuals who have recovered from infection.

Since the beginning of pandemic antibody based rapid tests have been used for rapid diagnostic. Most of the manufacterors do not provide details of the nature of antigen used. Also the test is a qualitative one which can determine only the absence and presence of infection but cannot give an idea about the viral load.

Summary:

Detectable and continuous high level of IgM indicated the acute phase of infection. 

IgM lasting more than a month indicating the prolonged virus replication in SARS-CoV-2 infected patients. 

IgG respond later than IgM and persisted high, indicating the humoral immune reaction to protect the body against SARS-CoV-2 virus.

Antibody testing has a role to play in supplementing PCR in diagnosis, screening of contacts and possibly in the determination of population immunity. 

Sensitivity varies with the stage of infection; it is low in the first week and then rises. Antibodies are highly specific. 

Combined PCR and antibody testing may be the optimal strategy for initial diagnosis given the dynamics of the infection and host response and the limitations of PCR testing. 

Significant questions remain with regard to the performance of individual test methods and the degree of immunity associated with the antibody response.