Dr Sujit Chatterjee – Chief Executive Officer
Dr. L.H Hiranandani Hospital
Though in the detection of the SARS-CoV-2 virus causing the COVID-19 infection, the gold standard of diagnosis remains the RT-PCR method, the serologic methods have been developed and will have important public health and clinical uses to monitor and respond to the COVID-19 pandemic.
Viral RNA, Antigen and Antibody response pattern:
Antibodies (IgG and IgM) most commonly become detectable 1-3 weeks after symptom onset, at which time evidence suggests that infectiousness likely is greatly decreased and that some degree of immunity from future infection has developed. IgG antibodies generally start appearing once the individual has recovered after infection and last for several months. Therefore, the IgG test is not useful for detecting acute infection.
Serologic tests detect waning or past SARS-CoV-2 virus infection indirectly, by measuring the host humoral immune response to the virus. There are several unanswered questions on various aspects of the immune response to the infection as the immunologic correlates of immunity from SARS-CoV-2 infection are not well defined.
- Kinetic of antibody response
- Level of antibodies required for protection from reinfection
- Duration of that protection
- Factors associated with development of a protective antibody response
Antigenic targets for Antibody Assay:
The two major antigenic targets of SARS-CoV-2 virus against which antibodies are detected are spike glycoprotein (S) and nucleocapsid phosphoprotein (N). While S protein is essential for virus entry and is present on the viral surface, N protein is the most abundantly expressed immunodominant protein that interacts with RNA. Multiple forms of S protein — full-length (S1+S2) or partial (S1 domain or receptor binding domain [RBD]) — are used as antigens. “S1” is the least “conserved” implying that the S1 protein is more unique (less conserved) in comparison to N or full-length S, thus, less likely to cause cross- reactivity with antibodies to the corresponding proteins of other coronaviruses. It follows that assays targeting S1 protein should show higher specificity.
Choosing an Assay
It is important to minimize false positive test results by choosing an assay with high specificity and by testing populations and individuals with an elevated likelihood of previous exposure to SARS-CoV-2.
The utility of tests depends on the sensitivity and specificity of the assays and the predictive values of a test. Positive predictive value is the probability that individuals with positive test results are truly antibody positive. Negative predictive value is the probability that individuals with negative test results are truly antibody negative. Positive and negative predictive values are determined by the percentage of truly antibody positive individuals in the tested population (prevalence, pre-test probability) and the sensitivity and specificity of the test.
For example: In a high-prevalence setting, the positive predictive value increases — meaning it is more likely that persons who test positive are truly antibody positive – than if the test is performed in a population with low-prevalence. When a test is used in a population where prevalence is low, the positive predictive value drops because there are more false-positive results, since the pre-test probability is low.
Likewise, negative predictive value is also affected by prevalence. In a high-prevalence setting, the negative predictive value declines whereas in a low-prevalence setting, it increases.
In areas where the prevalence of SARS-CoV-2 antibody is expected to be low (<5%), the testing might result in relatively more false positive results and fewer false-negative results.
In the current pandemic, maximizing specificity and thus positive predictive value in a serologic algorithm is preferred in most instances, since the overall prevalence of antibodies in most populations is likely low. To improve positive predictive value, the strategy is to choose a test with a very high specificity, perhaps 99.5% or greater, as this will yield a high positive predictive value in populations tested with prevalence of about 5%. Another strategy is to focus testing on persons with a high pre-test probability of having SARS-CoV-2 antibodies, such as persons with a history of COVID-19-like illness.
Currently, there is no substantive performance advantage of assays whether they test for IgA, IgM and IgG, or total antibody. Total Antibody tests can aid in diagnosis of acute or recent infection. IgG Antibody tests identify individuals for adaptive immune response, indicating recent or prior infection.
Indications for use of serology assay:
Detection of IgG antibodies for SARS-CoV-2 may be useful in the following situations:
- Serosurveys to understand the proportion of population exposed to infection with SARS-CoV-2 including asymptomatic individuals.
- Survey in high risk or vulnerable populations (health care workers, frontline workers, immunocompromised individuals, individuals in containment zones etc) to know who has been infected in the past and has now recovered
- To determine who may qualify to donate convalescent plasma: Serologic test results may assist with identifying persons potentially infected with SARS-CoV-2 and determining who may qualify to donate blood for convalescent plasma as a possible treatment for those who are seriously ill from COVID-19.
Source: https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antibody-tests-guidelines.html
