Why Do We Need Antibody Tests for COVID-19 & How to Interpret Test Results?
Diagnosing viral infections currently relies on two major methodologies: Reverse Transcription Quantitative Polymerase Chain Reaction (RT-qPCR) and serological immunoassays that detect viral-specific antibodies (IgM and IgG) or antigens.
Although, RT-qPCR is a highly sensitive test for SARS-CoV-2 (the virus that causes COVID-19) it has its limitations. RTqPCR requires high-quality nasopharyngeal swabs containing sufficient amounts of viral RNA. This can be a challenge because the amount of viral RNA not only varies tremendously between patients, it can also varies within the same patient depending on the timing of the test and the start of the infection and/or the onset of symptoms. In addition, nasopharyngeal swabs are not only very unpleasant to the patient, the sampling techniques vary significantly from nurse to nurse.
Without sufficient viral RNA RT-qPCR can return a false negative test result. RT-qPCR also requires highly trained personnel to perform complex RNA extraction steps and PCR. Normally, this would not be a problem when testing a few thousand samples. RT-qPCR becomes an issue when dealing with a global pandemic with potentially millions of people to test. This leads to delays in testing as medical facilities become overwhelmed with requests.
According to recent estimates, false negative results obtained with RT-qPCR are more common than initially thought (1). Some health care experts go as far as stating that, based on their own experience, one in three patients who has been infected with SARS-CoV-2 tests negative with the RT-qPCR method (1). False negative results can have devastating impacts on the current efforts to contain the SARS-CoV-2 outbreak as infected patients are mistakenly given the green light to return home, return to work and possibly infecting others. Relying solely on nucleic acid tests to diagnose SARS-CoV-2 is a risky strategy. As such, calls to add independent testing methodologies to complement RT-qPCR are becoming increasing louder.
IgG/IgM serological tests offer some advantages over RT-qPCR. Firstly, serological tests detect human antibodies (proteins belonging to the immunoglobulin class) which are known to be much more stable than viral RNA. As a result, IgM/IgG serological specimens are less sensitive to spoilage during collection, transport, storage and testing than RTqPCR specimens. Secondly, because antibodies are typically uniformly distributed in the blood, serological specimens have much less variations than nasopharyngeal viral RNA specimens and can be easily collected with minor phlebotomy discomfort to the patient. Thirdly, unlike RT-qPCR, serological tests can detect past infection because virus-specific antibodies (unlike viral RNA) can persist in the blood for several weeks/months after onset of symptoms.
IgM/IgG serological tests also have some limitations, mainly related to the slow pace of the human antibody response to SARS-CoV-2. Although, several studies are still on-going, SARS-CoV-2 antibodies may not be detectable before 3 days after onset of symptoms (or at least 7 to 10 days after infection) (3-4).
While IgM/IgG serological tests alone may not be enough to diagnose COVID-19, they can be a valuable diagnostic tool when combined with RT-qPCR (see section below). In addition, because of their scalability, serological assays can be used in large-scale, whole-population, testing to assess the overall immune response to the virus and identify asymptomatic carriers of the virus. Indeed, 20-80% of COVID-19 cases are estimated to be asymptomatic (2).
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: Clinical Significance of an IgM/IgG Serological Test Result
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 phase (3-5).
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 virus (3-5).
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.
(2) Li R, Pei S, Chen B, Song Y, Zhang T, Yang W, Shaman J2. Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2). Science. 2020 Mar 16. pii: eabb3221.
(3) Lauer, S. et al., 2020. The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application. Annals of Internal Medicine.
(4) National Health Commission of the People’s Republic of China, New Coronavirus Pneumonia Diagnosis and Treatment Program (Trial Version 7).
(5) To KK, Tsang OT, Leung WS, Tam AR, Wu TC, Lung DC et al. (2020). Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study. Lancet Infect Dis. 2020 Mar 23. pii: S1473-3099(20)30196-1.