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Antibody tests are not accurate during the first week of COVID-19

Antibody tests are not accurate during the first week of COVID-19

Last Reviewed : 12/28/2020
Antibody tests are not accurate during the first week of COVID-19

The accurate and fast diagnosis of COVID-19 is the cornerstone to isolate the affected people and limit the further spread of the infection. Multiple tests are being considered to diagnose and confirm COVID-19 as soon as the symptoms appear.

Currently, RT-PCR test is the commonly used test to confirm the diagnosis. However, accuracy (sensitivity) of this test to detect COVID-19 is only around 70%. The accuracy of RT-PCR test depends on multiple factors like the skill to collect the sample, infection stage of the patient, and the quality and consistency of the test reagents being used.

Compared to RT-PCR test, antibody tests are faster, more reliable, less expensive, and not many skills required to collect the sample. A study published in the journal Clinical Infectious Diseases was conducted on 173 COVID-19 confirmed patients to determine the accuracy of the antibody tests. The median time to detect IgM antibody was 12 days and the median time to detect IgG antibody was 14 days.

According to this study, the sensitivity or accuracy to detect COVID-19, for RT-PCR and antibody tests are as below:

                                  

                                                                                Days after onset of symptoms

Test type

1-7 Days

8-14 Days

15-39 Days

RT-PCR

67%

54%

45%

Total Antibody

38%

90%

100%

IgM Antibody

29%

73%

94%

IgG Antibody

19%

54%

80%

RT-PCR + Total Antibodies

79%

97%

100%

 

 

In the first week of COVID-19 illness, the accuracy of antibody tests is below 40%. RT-PCR still seems to be the best test during the first week of course of the disease. The combination of antibody test and RT-PCR has a sensitivity of 79% but it is still not impressive. The accuracy of antibody tests is significantly higher in the second week. After two weeks of onset of symptoms, antibody tests accuracy reaches 100%.

Another study published by Xiang F et al. demonstrated that antibodies might be detected as early as 4th day after the onset of symptoms in a few patients. However, IgM antibody detection increased quickly only from the 9th day in most of the patients. Similarly, IgG detection increased from the 11th day after onset of symptoms.

In summary, based on these studies, antibody tests during the first week of COVID-19 are not helpful. The accuracy of antibody testing increases significantly during the second week. If RT-PCR test is negative, antibody test may be used as an alternative during the first week. During the second week, antibody test should probably replace RT-PCR as accuracy reaches 90%.

 

References:

  1. Zhao J et al. Antibody responses to SARS-CoV-2 in patients of novel coronavirus disease 2019. Clin Infect Dis. 2020 Mar 28. pii: ciaa344. doi: 10.1093/cid/ciaa344.
  2. Li Z et al. Development and clinical application of a rapid IgM-IgG combined antibody test for SARS-CoV-2 infection diagnosis. J Med Virol. 2020 Feb 27. doi: 10.1002/jmv.25727.
  3. Xiang F et al. Antibody Detection and Dynamic Characteristics in Patients with COVID-19. Clin Infect Dis. 2020 Apr 19. pii: ciaa461. doi: 10.1093/cid/ciaa461.

 

 

 

 

 

 

 

 

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