coronavirus

Work resumption is taking place across China. As ordinary a citizen, how do you choose your own testing method? What is the difference between the nucleic acid test and antibody detection?

Now let’s dive deeper into these detections, and carefully distinguish their respective traits.

nucleic acid test

It all starts when the virIus enters the human body, which is a type of pathogen, simple with only a protein shell wrapped with nucleic acids.

The simple structure makes it easier for viruses to break through the layers of defence of the human immune system, leading to infection.

The virus is so cunning, how do we judge the patient is infected?

A viral test not only detects antigens (generally pathogen surface proteins, some are internal nuclear proteins) but also nucleic acids.
During the outbreak of this pandemic, doing a viral nucleic acids test by PCR technology has become a major contributor to the detection of pathogens.

If a test is positive, novel coronavirus (SAR-cov-2) infection is confirmed.

In this way, if the nucleic acid test result is negative, can we assume that there is no infection?

The answer is no in that the cunning virus will not easily make you see the truth. Some patients negative will continue to infect others.

This situation is called “false negative”, and there are many possibilities for “false negative”, such as poor sample quality, improper sample storage and transportation or handling, viral mutation, PCR inhibition …

Well, desperate times, desperate measures, that is when CT examinations and serological tests come in handy.

The serological test is like a “boss” in the field of diagnostics, including the detection of various antibodies. One of them has to be mentioned is the immunoglobulin family (IgA, IgD, IgE, IgM and IgG). Under normal circumstances, they lead to the protection of the body against foreign pathogens.

For the novel coronavirus, the active players are IgM and IgG.

IgM, an offensive player, is the body’s anti-infection “first team”, generally as a scout. It looks like a small umbrella (secreted IgM pentamer structure). Once tested positive, it can be judged that the infection has occurred recently.

IgG, an auxiliary player, is characterized by a large number of people, strong for long-term tug of war, and looks like a double-ended toothbrush. If it is defeated (tested positive), it can be judged as the previous infection, or in the middle and late stages of infection.

Studies have shown that after SAR-Cov-2 enters the body, IgM antibodies can be produced in 5-7 days, and IgG antibodies are produced in 10-15 days.

So as a supplement to the nucleic acid test, how to interpret the results of antibody detection?

When the nucleic acid test result is positive, the results of the serological antibody test are mainly shown below:

Interpretation of antibody test results when nucleic acid test is positive

When the nucleic acid test result is negative, the results of the serological antibody test are mainly shown below:

Interpretation of antibody test results when nucleic acid test is negative

Features of Nucleic Acid Test and Serological Test

Nucleic Acid Test

Highly accurate, demanding, and long time-consuming, suitable as the first line of defence; capable of discovering infected people in the window period;

There are higher requirements for the detection platform and personnel. If the test result of the suspected patient is negative, the infection cannot be ruled out, and a follow-up test is required.

Serological Test

Simple, fast, generally accurate, suitable as an auxiliary; specimens are easy to obtain, easy to operate, and short time-consuming; however, it takes time to produce antibodies after the antigen enters the body, with a window period.

No matter what method is adopted for testing, the performance of reagents produced by different manufacturers may also be different. Therefore, in practical work, the serological test is often considered an important supplementary means for the nucleic acid test. This helps to screen out the patients with SAR-Cov-2 infection among suspected patients, especially those with negative nucleic acid test results. In clinical work, it is often a comprehensive application of multiple technical methods to assist diagnosis and improve the efficiency of epidemic prevention and control.


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