Serology

Laboratory Testing Recommendations for COVID-19 Serology Based Assays

Background: The antibody testing or use of immunoassays are NOT RECOMMENDED FOR ROUTINE COVID-19 DIAGNOSIS. The potential diagnostic applications for immunoassays may include estimation of the disease stage (acute, early, convalescent) in combination with RT-PCR; assessing the immune status of healthcare and other essential personnel; assessing convalescent plasma and vaccine efficacy (more suitable for ELISAs versus RDTs); and guiding identification of low-risk people to exit quarantine (e.g. lifting lockdown measures). Some of the developed countries have allowed immunoassays/serological testing and have diverse experience, however, most of the settings are using such assays for sero-epidemiological studies at community level and for public health research. Moreover, public health decision making depends on understanding disease dynamics including prevalence of immunity to COVID-19, which in turn is achievable through the serological testing and guide prioritisation of vaccine as it becomes available. However, even though such assays also provide valuable information, there is significant uncertainty about the performance and validation of these assays.

Timeline for SARS-CoV-2 Testing:

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Performance characteristics for immunoassays include:  

Analytical sensitivity – also known as limit of detection, the lowest concentration of antigen or antibody that the test can detect.

Analytical specificity − or cross-reactivity, which looks at whether the test will detect other biomarkers (including other coronaviruses) that are not SARS-CoV-2, along with other substances that could interfere with the test.

Factors affecting the performance of serology tests:  

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Cross-reactivity to other coronaviruses Timing of sample collection will affect the results as it generally takes 7- 10 days to build up a strong antibody titre/response. Prior testing may lead to false negative results. It may be helpful to collect a follow-up sample for antibody testing a few weeks after the first, in case the first result was collected too early. In addition, if using an ELISA, it can be seen if there is a change in the amount/titre of antibody for individuals who test positive.

Type of test to be used: Because of the simplicity of test design, RDTs may be less sensitive than laboratory-based ELISAs.

Rapid Diagnostic Tests (RDTs): There are no rapid diagnostic tests (immuno-chromatography or colloidal gold detection) that have been authorized by WHO and have been systematically validated by global technical agencies. In general, these types of tests have low sensitivity. Their positive predictive value is good (can be used to rule in the cases), but their negative predictive value is low (should not be used to rule out cases). Also, the limitations described above for serological tests and antigenic detection apply to RDTs. Laboratory Based Serological Methods (ELISA): Several laboratory based assays are available for the detection of IgM/IgG antibodies and are marketed for the detection of COVID-19 virus infections, with emergency use authorization.

However, their use alone for diagnostic purposes is generally not recommended for the following reasons: a. Serological tests are not diagnostic/confirmatory tests due to varying degree of sensitivity and specificity. b. During the first 6-7 days from the onset of symptoms, less than 40% of patients have detectable level of antibodies (IgM and IgG). Thus, serological tests should not be used to rule out a case during the early days of illness. c. Detection of antibodies after day 7 only indicates previous contact with the virus but does not confirm presence and shedding of the virus. d. Serology based negative tests should be backed up with PCR testing.

Considerations for Serological Antibody Testing: a. For community screening and research purposes. b. For individual cases, serological test can be done to see the evidence of exposure to SARSCoV-2.

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c. At community level, serological tests may be used for sero-epidemiological or seroprevalence studies to inform public health measures. d. For convalescent plasma transfusion purposes. e. Laboratory based serology tests may be considered as supplementary tests to RT-PCR based testing. f. Regulatory considerations such as in vitro diagnostics with at least one of the following recommendations are applicable for all facilities that undertake serological testing: - EU regulatory status such as CE marked - FDA approved/notified

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