Antigen and Antibody Detection
Serology helps in the investigation of the ongoing pandemic especially in cases where NAAT assays are found to be negative and the link between illness (clinical manifestations) and COVID-19 is very strong, so that samples can be collected for both acute and convalescent phases for serology (WHO, 2020).
In response to SARS coronavirus infections, different types of immunoglobulins (antibodies) including IgG and IgM are produced and can change in level in the course of infection. The antibodies can be undetectable at the initial infection stage; IgG can be detected even after the illness has been resolved. The antibody tests for the disease include ELISA to detect both IgG and IgM antibodies which is more reliable especially 21 days of infection, and Immunofluorescence assay to detect only IgM or IgG antibodies after about 10 days of infection. Positive antibody test signifies infection whereas the reverse indicates no infection has taken place (WHO, 2003). Immunoblot is another serologic technique for SARS-CoV antibodies (Maschinen et al., 2005).
Serologic techniques for identification of COVID-19 antibodies including IgA, IgG and IgM from clinical specimen such as ELISA are less reliable than molecular tests and can potentially be utilised for early diagnosis. There is limitation around the onset of the symptoms when incubation and transmission are high, and the body may not likely start producing antibodies. The response of antibodies to the viruses usually takes many days or weeks to be clearly detected, and, negative outcome do not count out infection particularly at early stage (Cheng et al, 2020; USFDA, 2020). Cross reactivity with antibody to non COVID-19 proteins must also be put into consideration, so that positive result may be as a result of recent or past infection with other coronaviruses. Serology is more relevant in a situation whereby patient present with complication of late disease when RT-PCR is likely to produce false negative result as a result of dropping of viral load over time (Cheng et al, 2020).
Detection of SARS-CoV directly using ELISA have not been possible, rather, its nucleocapsid protein from respiratory specimen, faeces and urine; potential nucleocapsid protein has been used as reliable diagnostic tool to detect the virus (Lau et al., 2004). ELISA was known to be highly specific, less expensive and labour intensive than RT-PCR. Western blotting (WB) and IF assay have also been used to detect serum SARS-CoV (Leung et al., 2004). Detection of influenza virus antigen by IF directly from clinical sample have been available and is less complex, providing results at point of care but have suboptimal sensitivity to rule out disease, this challenge may exist probably in the case of COVID-19, therefore implementation of test of this nature need clear guidance on correct interpretation.
It was already recommended that negative serologic results do not rule out SARS-CoV-2 infection, rather molecular testing should be conducted particularly on samples of highly suspected individuals, furthermore, the results of antibody testing should not be relied on for confirmation or exclusion of COVID-19 infection or its status as positive outcome may be as a result of present or fast encounter with other non COVID-19 coronavirus strains (USFDA, 2020). Monoclonal antibodies against nucleocapsid of 2019-nCoV is currently generated for future antigen detection test (Cheng et al., 2020). Recombinant spikes protein (S-protein) and nucleocapsid protein (N-protein) are currently in use to manufacture diagnostic kit for 2019-nCoV serum antibodies (Xu, 2020).