Antibody Test for Detecting Novo Coronavirus(SARS-CoV-2)

The epidemic of COVID-19 outbroke by the end of 2019 and has developed as pandemic all over the world, which brings great pressure for the healthcare system in many countries. The novo coronavirus (SARS-CoV-2) is highly infectious even at the early stage of infection, so effective tests and early diagnosis of infected patients is very important. At present, nucleic acid test and Serology test (antibody test) are most common methods for detecting SARS-CoV-2.

Nucleic acid test (NAT, such as RT-PCR) detects the RNA of SARS-CoV-2 which is a part of the virus itself, while serology test detects antibodies against SARS-CoV-2 which is produced by the immune system of COVID-19 patients. When one is infected with the virus, the nucleic acid can be detected first, followed with the antibodies. Nucleic acid appears quite early and is very specific, so NAT is widely accepted as a confirmatory test for the diagnosis of COVID-19, but it still has some limitations.

(1)Sampling problem: The throat or nasal swab, sputum and alveolar lavage fluid are sample types commonly used for NAT. Though the result of NAT is very specific, it’s highly affected with the sampling process. SARS-CoV-2 mainly exists at the lower respiratory tract, so the sample collected at the upper respiratory tract may contain little or even no virus. The detection rate with alveolar lavage fluid sample might be higher, but the procedure of collecting alveolar lavage fluid is usually too complicated and dangerous to be put into practice.  Nonstandard operation of sampling can also cause false negative result.

(2)False negative result caused by the disease course: With extension of the disease course, the patient’s immune system is mobilized to produce antibodies to attack the SARS-CoV-2 virus. This may inhibit the duplication of virus and cause false negative result of NAT.

(3)High requirement for labs and operation staff: It is required to operate the NAT of SARS-CoV-2 in labs with biosecurity level higher than P2, and the operation staff should also be well trained. Moreover, for most NAT kits, the operation time can be as long as 3-4 hours. The test reagents of NAT should be kept at -20℃, which also causes difficulty for transportation and storage.

(4)Quality Issue of test kits: There are many suppliers who produce NAT kits with different quality. Products with poor quality my cause unreliable test results.

    Because of the limitations of NAT mentioned above, it is necessary to introduce antibody test as a supplement to NAT for the diagnosis of COVID-19. 

When one is infected with SARS-CoV-2, his immune system will be stimulated to react with the virus and produce specific antibodies. The first antibody produced is IgM, it normally appears 3-5 days after the disease onset and rises to its peak level in 1-2 weeks, then it begins to go down and disappears gradually. IgG antibody may appear 1-2 days after the arise of IgM,its concentration will continuously rise and lasts for years. Once the antibodies are produced, they can exist steadily in blood. Therefore, when a COVID-19 patient is tested as negative with NAT, antibody test can be a good supplement.. 

IgM antibody appears early and disappears quickly, so it is a marker for early infection. IgG antibody appears a little bit later, but lasts for quite long time. The result pattern of antibody test is as follows::

  • IgM and IgG are both positive:The patient is most probably be infected with SARS-CoV-2, combined with the related symptoms, one can be diagnosed as the case of COVID-19.
  • IgM positive and IgG negative: It might be an early infection case whose antibodies should be tested and monitored continuously. If IgG converts from negative to positive, the patient can be diagnosed as the case of COVID-19; if IgG is repeatedly tested as negative, the IgM may be indicated as false positive.
  • IgM negative and IgG positive:It might be a late infection case and the patient is getting recovery. In such case, the IgM disappears and IgG stays positive. However, if IgG always shows weak positive result, it might be false positive.
  • IgM and IgG are both negative: It can be either a normal person or an early infection case whose antibodies are not produced yet, so double negative result cannot exclude the possibility of SARS-CoV-2 infection. The patient should be monitored continuously, if both antibodies are still negative after one week, the infection can be excluded.

Conclusion:Antibody test is simple, rapid and easy to use, the test result can be obtained in 15 min, therefore, it is suitable to be used in all level hospitals or clinics. A combined test of IgM and IgG can not only indicate the false positive result that might be produced by a test for single antibody, but act as a supplement to NAT as well. Furthermore, with continuous monitoring of both IgM and IgG, the regulation of antibodies can be observed which is very valuable for distinguishing the disease course of COVID-19.

Source : Livzon Diagnostics (04/2020)

Catégories : Knowledge base

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