In December 2019, several cases of pneumonia of unknown etiology, now known as coronavirus disease 2019 (COVID-19), occurred in Wuhan, China, has been recognized as a pandemic by the World Health Organization (WHO) [1, 2] . The disease has rapidly spread from Wuhan to other provinces in China and around the world. As (SARS-CoV-2)" as the name of the new virus caused COVID-19 [3] .
Early detection, early reporting, early isolation, early diagnosis and early management have been proved to be useful in the course of COVID-19 patients [4] .
But how to detect early was an unmet demand in front of medical personnel. It is well known globally that virus nucleic acid high throughput sequencing or virus nucleic acid RT-PCR assay was the standard and recommended methods for detecting SARS-CoV-2, which caused COVID-19 pandemic across the whole world. High throughput sequencing can act as the 'golden standard' but it is highly restricted by its resource-dependent platform, especially in those resource-limited settings. Virus nucleic acid RT-PCR assay also suffer some limitations, long turnaround time, requirements for certified laboratories, skilled technicians and expensive equipment, and especially false negative results [5] . Generally speaking, RT-PCT results can be affected by many factors, such as quality of samples, degeneration of virus RNA, and improper sampling of nasal and pharyngeal swab. A former study revealed that 5 cases who were continuously PCR negative but with chest CT scan positive results were finally PCR confirmed positive after a series of continuous RT-PCR testing, which will inevitably hinder COVID-19 pandemic control and limit the outbreak containment effort [6] . This was in agreement with similar studies related to SARS in 2003, whose results showed that RT-PCR positive rate was only between 50% and 79% [7] . In addition, the number and variety of diagnostic tests should be expanded . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. (which was not certified by peer review)
The copyright holder for this preprint this version posted March 30, 2020. . https://doi.org/10.1101/2020.03.26.20044883 doi: medRxiv preprint based on the fact that severity and scope of the current COVID-19 situation around the globe necessitates greater testing capacity than is currently available. Therefore, detection methods that are faster and more convenient, and complementary to nucleic acid detection, are particularly important in the current epidemic prevention and control. Hence detecting antibodies of SARS-CoV-2 became a supplemented option for RT-PCR, no matter IgM or IgG or combined, which can provide another key piece of evidence for the diagnosis of viral infections.
The New Coronavirus Pneumonia Prevention and Control Program (7th edition) published by the National Health Commission of China also recommended that any suspicious cases together with positive IgM/IgG antibodies can be deemed as positive COVID-19 cases. However, data concentrating on its sensitivity, specificity were inadequate, which were of paramount value in the decision whether, when and how to use IgM/IgG antibodies detection. The aim of this study was to evaluate the sensitivity, specificity, accuracy, positive predictive value (PPV), negative predictive value (NPV), especially Kappa efficiency with PCR and its anti-interference ability. Data analysis SPSS 20.0 was used for statistical analysis All tests were bilateral, and p less than 0.05 was considered statistically significant. Chi-square test or Fisher exact probability test was used to compare count data. Sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and Kappa efficiency were calculated.
Just as described in Method section, C line should appear in any test, otherwise the test is invalid. Either G or M or both lines appearing red lines shows presence of anti-SARS-CoV-2 IgG or anti-SARS-CoV-2 IgM or both antibodies in this detected sample. A representative picture for five patients blood testing results was shown in . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this preprint this version posted March 30, 2020. Result of patient 4 was positive for both IgM and IgG antibodies. 
PCR positive group was further divided into two subgroups: mild/common and severe/critical subgroup while PCR negative group was further divided into three subgroups: clinical confirmed, suspected cases and other disease subgroups ( Table 2) .
Because PCR result in each subgroup was constant, diagnostic indexes except . CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this preprint this version posted March 30, 2020. 
According to a previous study, IgM antibody could be detected in patient blood after 3 -6 days of SARS infection while IgG antibody could be determined after 8 days of SARS infection [8] . SARS-CoV-2 which caused COVID-19 belong to the same large family as that caused SARS [9] , hence it is very important to calculate diagnostic indexes of this test kit stratified by the time from illness onset or infection time to sample collection.
A total of 115 inpatients, including 90 PCR confirmed positive cases, 5 clinical confirmed cases and 20 suspected cases, were divided into three groups according to the time from illness onset to sample collection: '0 -7 days' group, '8 -15 days'
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The copyright holder for this preprint this version posted March 30, 2020. 
In the past week, COVID-19 has started behaving a lot like the once-in-a-century pathogen that international experts have been worried about [10] . Because the symptoms of COVID -19 are not unique but similar to those of other diseases, testing is the only way to know for sure if someone is infected with the SARS-CoV-2 [11] .
Mass testing is therefore of paramount importance to curb SARS-CoV-2 epidemic.
Virus nucleic acid RT-PCR is not suitable for large scale screening owing to the inherent properties. On the contrary, the qualitative detection of SARS-CoV-2 IgG/ IgM antibodies in human serum, which is configured like a home pregnancy test, has the ability for mass testing.
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The copyright holder for this preprint this version posted March 30, 2020. The ease-of-use IgG/IgM combined test kit under study was developed based on gold immunochromatography assay (GICA), which can generate results in less than 15 minutes from human serum without laboratory equipment or skilled personnel or sample transportation. Its diagnostic indexes or so-called test characristics, including sensitivity, specificity, PPV, NPV and accuracy were evaluated to determine its diagnostic usefulness [11] . Our results revealed that the above-mentioned five indexes were 85.6%, 91%,95.1%, 82.7%, and 88.3%, respectively which was in agreement with a recently published research [5] . In that study, sensitivity and specificity were 88.66% and 90.63%. Sensitivity is the ability to diagnose those with the disease correctly identified as positive by the test. From the definition of sensitivity, we can suggest that manufacturer should try to improve IgG/IgM test kit's detection sensitivity. Because the lower the sensitivity, the more false-negative cases.
False-positive cases can be further confirmed by other detection methods; hence false-negative cases would pose more opportunities to infect people they contact. CC-BY-NC-ND 4.0 International license It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this preprint this version posted March 30, 2020. . https://doi.org/10.1101/2020.03.26.20044883 doi: medRxiv preprint examination after one week or so was strongly suggested. Kappa efficiency was also investigated in our study (Kappa = 0.75, p < 0.001), which inflected moderate agreement between IgG/IgM test kit and RT-PCR assay.
Accuracy of SARS-CoV-2 IgG/IgM test kit in 5 clinical confirmed cases showed 2 out of the 5 cases were tested positive by IgG/IgM test kit while their RT-PCR results were negative ( Table 2) . SARS-CoV-2 infection starts at the lung not in the upper respiratory tract, hence sampling process has a large effect on final RT-PCR results [13] , which might partially explain the high false-negative rate. However, this should not have any effect on SARS-CoV-2 IgG/IgM test kit because only venous blood was needed for this test. IgG/IgM test kit likely can remedy some false negatives inherent in respiratory swab samples and can be served as a complementary option to RT-PCR.
Compared with other similar studies, one of our strengths was that we recorded detailed time of each patient from illness onset to sample collection. From the results (Table 3) , we can see that only 3 out of the 16 cases with positive PCR results in '0 -7' group were tested positive by SARA-CoV-2 IgG/IgM test kit, generating a sensitivity of just 18.8% (3/16). The time from illness onset to sample collection of these 16 patients in '0 -7' group was between 1 and 2 days. Patients in this group may be at the initial stage or 'window period' of SARS-CoV-2 infection, the concentration of antibodies was too low to be detected. In '8 -15' subgroup and '>= 16' subgroup, the sensitivity of IgG/IgM test kit jumped from 18.8% to 100% whereas the specificity in these two groups were relatively low, 50% and 64.3%, respectively.
It was very difficult to explain, because we were unsure the low specificity was caused by false negative results of RT-PCR, taking high false negative rate of RT-PCR into consideration, or by false positive results of IgG/IgM test kit itself or by small sample size. The other strength of our study was enrollment of 34 patients diagnosed with autoimmune disease, such as Sjogren's syndrome, systemic lupus erythematosus, rheumatoid arthritis, and connective tissue disease. The enrollment of these patients was to determine IgG/IgM test kit's anti-interference ability. Our results showed IgG/IgM test kit performed well in these patients.
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The copyright holder for this preprint this version posted March 30, 2020. However, some limitations also should be pointed out. First, due to the limited time, diagnostic indexes were only evaluated in serum, but not other blood sample types, such as fingertip blood and plasma. Second, small sample size should also be taken into consideration.
The sensitivity and specificity of this ease-of-use IgG/IgM combined test kit were good, plus short turnaround time, no specific requirements for additional equipment or skilled technicians, all of these can collectively contributed to its competence for mass testing. At the current stage, it cannot take the place of SARA-CoV-2 nucleic acid RT-PCR, but can be served as a complementary option for RT-PCR. The combination of RT-PCR and IgG/IgM combined test kit could provide further insight into SARS-CoV-2 infection diagnosis.
The authors declare that they have no competing financial interests.