In this retrospective study, we reviewed SARS-CoV-2 IgM/IgG antibody and RT-PCR assays of 103 cases with a moderate type of COVID-19 for the purpose of understanding the progression of COVID-19, the diagnostic utility of these assays, and ultimately finding better treatment strategies. According to the national guidelines, RT-PCR is considered one of the main diagnostic tools for COVID-19, but the RT-PCR assay for COVID-19 has a false negative rate of approximately 50%. Hence, the sensitivity of the RT-PCR assay is poor in clinical practice, and necessary measures need to been taken, such as repeated RT-PCR testing or collecting multiple samples from the same subject, to better understand a patient’s status. Furthermore, a RT-PCR (-) result should not serve as the only criteria for determining patient discharge; evaluation of clinical symptoms and imaging, as well as antibody testing are also required . We conducted this retrospective study to further improve our understanding of RT-PCR status in conjunction with variations in IgM/IgG antibody levels for determining clinical course and patient discharge. To the best of our knowledge, this is the first report investigating the correlation between RT-PCR status and IgM/IgG antibody levels in COVID-19 patients.
In this study, we observed that the SARS-CoV-2 IgM levels peaked on day4 and remained positive 21 days after the RT-PCR (-) result. The peak IgG level was approximately 20-fold greater than that of the upper reference at day4. The IgM levels decreased over time. Yu Chun et al, reported that the neutralizing antibody (NAb) against SARS-CoV was found in 85.9% of COVID-19 cases, and that most prevalent immunoglobulin class was IgG. NAbs typically become detectable 5-10 days after the onset of symptoms. The NAb levels usually peak at 20-30 days and remain at these levels for > 150 days . IgM levels are usually first detected at 20.5 days after infection, peak 80 days later, and then fall to baseline levels at 180 days . In another SARS study , the conversion of IgG antibody (approximately 10 days after infection) occurred simultaneously and earlier than that of both IgM and IgA (approximately 11 days). IgG was detected as early as 4 days after the onset of illness, with the earliest detection of all three antibodies reaching peak levels at 15 days. Elevated IgG levels can persist for more than 3 months .
Yang et al, showed that in COVID-19 patients, the sensitivity of virus RT-PCR using sputum and nose swabs was higher than that of pharynx swabs at 0-7 days, 8-14 days, and ≥ 15 days, respectively . With the progression of the disease, the rate of pharynx swab RT-PCR(+) results decreased significantly (from 61.3% at 0-7 days to 11.1% at ≥ 15 days) . Hence, nucleic acid extraction and disease course can impact RT-PCR results. The COVID-19 IgM/IgG antibody assay has been shown to have good diagnostic performance for detecting infection. As such, the COVID-19 IgM/IgG antibody assay has been developed and recommended for clinical practice as a complement to standard RT-PCR testing. The use of this antibody assay may reduce the impact of COVID-19 false negative RT-PCR tests due to inappropriate extraction and collection methods (nasopharyngeal swab sample). In our study, the sensitivity of COVID-19 antibodies (IgM and/or IgG) for detecting infection was 96.83% (61/63), with only 2 COVID-19 patients testing negative after the IgM/IgG antibody assay among the 63 tested patients. This result is consistent with a previous finding , which reported a sensitivity of RT-PCR at only 30 - 50% . Considering its diagnostic potential, the SARS-CoV-2 IgM/IgG antibody assay can be used to screen COVID-19 infection, monitor clinical course, determine discharge, and assess recovery. In addition, the RT-PCR for COVID-19 faces several challenges. For example, it is thought that the global supply is difficult to meet a huge demand for the PCR primers and positive controls. Hence, the antibody assay may be helpful as an alternative method in screening patients with COVID-19 . Fortunately, in a recent study by Wang To KK, et al., posterior oropharyngeal saliva samples, as a non-invasive specimen for RT-PCR test, is proved to be a good choice for serodiagnosis of COVID-19, due to a high sensitivity .
The IgM/IgG antibody assay has several advantages over the RT-PCR assay for diagnosing COVID-19. First, RT-PCR requires respiratory samples acquired from a swab, sputum, and/or bronchoalveolar lavage fluid (BALF), and the assay must be conducted in a strictly regulated biological safety facility. The IgM/IgG antibody assay requires only a serum or plasma sample. Second, the false-negative rate of the antibody assay is much lower than that of RT-PCR assay, which will facilitate more rapid and more sensitive in identification of COVID-19 patients. It is supposed to be urgent values for large number of infected people screening, detecting, immune status evaluating, which would be useful in tracing suspected infection, guiding people back to work(including infected health-care workers getting back to work with immune), reopening the lockdown, finding asymptomatic infection and so on . Third, the antibody assay used in our study is a qualitative biomarker, which will be useful for monitoring patient progression and outcome after infection. Fourth, the antibody test can be used to verify the effectiveness of new vaccine .
In our study, the findings supported that the level of lymphocyte count was positively correlated with viral load. When infected with the novel virus, the level of lymphocyte count was low, such as a lowest level occurs on day7 before the RT-PCR(-) status. But, under the circumstance of viral clearance and improvement of disease status, the lymphocyte count increased gradually. The variation of lymphocyte count may also be taken as a monitor biomarker for the management of COVID-19 patients and the normal level of lymphocyte could be thought as a recovery sign of COVID-19.
Our study found that in patients with moderate type of COVID-19, there was no statistically different relationship between IL-6 level and the course of disease. This finding is inconsistent with previous data reported by other literatures [15,16]. This may result from a moderate inflammatory reaction in moderate type of COVID-19, and further analysis should be performed in severe cases.
Our study does have some limitations that should be noted. First, only patients with a moderate type of COVID-19 were enrolled, and the data for severe infections remains uncertain. Second, due to the retrospective design, information about viral load in the nasopharyngeal swabs is not known. Thus, correlation analysis between viral load and IgM/IgG levels was not attempted. In the future, the time for IgM and IgG reversion should be evaluated carefully. In addition, the association between IgM/IgG levels and the recovery efficiency should also be investigated in a prospective manner.