Diagnostic Indexes of a Rapid Immunoglobulin G/Immunoglobulin M Combined Antibody Test for Severe Acute Respiratory Syndrome Coronavirus 2
The detection of antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) serves as a supplementary method to reverse transcription polymerase chain reaction (RT-PCR) testing. This includes the detection of immunoglobulin M (IgM), immunoglobulin G (IgG), or a combination of both. However, there is insufficient data on the sensitivity and specificity of IgM/IgG antibody detection, which is crucial for determining when and how to use these tests. This study aimed to evaluate the sensitivity, specificity, accuracy, positive predictive value (PPV), and negative predictive value (NPV) of IgM/IgG antibody detection, particularly in comparison to SARS-CoV-2 RT-PCR, which was used as the standard diagnostic method.
The study enrolled 179 participants who visited or were admitted to the General Hospital of the Central Theatre Command between January 1, 2020, and March 12, 2020. Blood samples were collected from all participants for SARS-CoV-2 IgG/IgM antibody detection using a test kit from Innovita Biological Technology Co., Ltd. Nasal or pharyngeal swab specimens were also collected for RT-PCR testing by DAAN Gene Co., Ltd. A confirmed COVID-19 case was defined as a patient with clinical symptoms, including acute respiratory infection syndromes and/or abnormalities in chest computed tomography images, and detectable SARS-CoV-2 RNA in respiratory samples at least once since the onset of illness.
The participants were divided into two groups: a confirmed COVID-19 group (90 cases) and a non-confirmed COVID-19 group (89 cases). The confirmed group included 46 mild/common cases and 44 severe/critical cases, while the non-confirmed group comprised five clinically confirmed cases, 20 suspected cases, and 64 cases with other diseases, including autoimmune disorders and common injuries.
The SARS-CoV-2 IgG/IgM test kit demonstrated a sensitivity of 85.6% (77/90) in the confirmed group and a specificity of 91.0% (81/89) in the non-confirmed group. The PPV, NPV, and accuracy of the test kit were 90.6% (77/85), 86.2% (81/94), and 88.3% (158/179), respectively. The kappa coefficient between the IgG/IgM test kit and the standard RT-PCR diagnosis was 0.75, indicating moderate agreement between the two methods.
The study further stratified the confirmed group into mild/common and severe/critical subgroups, and the non-confirmed group into clinically confirmed, suspected, and other disease subgroups. The accuracy of IgG/IgM detection varied across these subgroups, ranging from 60% to 100%. The test kit performed poorly in the “0 to 7 days” group after illness onset, with a sensitivity of 18.8% (3/16), specificity of 77.8% (7/9), and accuracy of 40.0% (10/25). However, it performed best in the “≥16 days” group, with a sensitivity of 100.0% (68/68) and an accuracy of 93.9% (77/82), albeit with a lower specificity of 64.3% (9/14).
The study highlighted that IgM antibodies can be detected in patient blood 3 to 6 days after SARS-CoV infection, while IgG antibodies can be detected 8 days after infection. Given that SARS-CoV-2 belongs to the same family of viruses as SARS-CoV, it is essential to evaluate the diagnostic performance of the test kit at different stages of infection. The results showed that the test kit’s sensitivity improved significantly as the time from illness onset increased, but its specificity remained relatively low in later stages.
The study also emphasized the importance of mass testing to curb the SARS-CoV-2 epidemic. While RT-PCR is not suitable for large-scale screening due to its technical requirements, the qualitative detection of SARS-CoV-2 IgG/IgM antibodies in human serum offers a more accessible alternative. Positive IgM results suggest recent or early-stage infection, while positive IgG results indicate past or late-stage infection. Therefore, combined detection of IgG and IgM antibodies is recommended for different stages of COVID-19.
The diagnostic indexes of the IgG/IgM test kit, including sensitivity, specificity, PPV, NPV, and accuracy, were evaluated to determine its diagnostic usefulness. The results showed that these indexes were 85.6%, 91.0%, 90.6%, 86.2%, and 88.3%, respectively, which aligns with findings from a recently published study. The study suggested that the manufacturer should focus on improving the sensitivity of the test kit to reduce false-negative results, which could lead to further transmission of the virus.
The PPV of the IgG/IgM test kit was 90.6%, indicating that 90.6% of individuals with a positive test result were likely to have the disease. Conversely, the NPV was 86.2%, meaning that 86.2% of individuals with a negative result were likely disease-free. However, the study cautioned that negative results should not exclude the possibility of infection, and repeat testing after approximately one week is recommended.
The study also addressed the issue of false-negative RT-PCR results, which may occur due to the sampling process. Since SARS-CoV-2 infection starts in the lungs rather than the upper respiratory tract, the IgG/IgM test kit, which requires only venous blood, could serve as a complementary option to RT-PCR. The test kit’s performance in autoimmune disease patients was also evaluated, showing no significant interference, which further supports its diagnostic utility.
Despite its strengths, the study acknowledged several limitations. First, the diagnostic indexes were evaluated only in serum samples, and not in other blood sample types such as fingertip blood or plasma. Second, the small sample size may have affected the results. Third, further studies with larger populations are needed to provide more insights into seroconversion and the optimal timing for antibody detection.
In conclusion, the rapid IgG/IgM combined antibody test kit demonstrated good sensitivity and specificity, with a short turnaround time and no need for specialized equipment or skilled technicians. While it cannot replace SARS-CoV-2 nucleic acid RT-PCR, it serves as a valuable complementary tool for mass testing and diagnosing COVID-19 at different stages of infection.
doi.org/10.1097/CM9.0000000000001204
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