How to decipher the test for antibodies to coronavirus

2024 Author: Malcolm Clapton | [email protected]. Last modified: 2023-12-17 03:44

Don't confuse it with a coronavirus test (PCR).

To check whether a person is infected with a coronavirus and whether he can infect others, the PCR test is mainly used - using the polymerase chain reaction technique. To do this, using a cotton swab, take a swab from the nasopharynx or throat and examine the resulting saliva sample, trying to find the causative agent of the disease in it.

An important feature of the PCR test: it helps to establish whether a patient is suffering from a coronavirus infection right now.

But if you want to know if you have had COVID-19 before - without symptoms or in the form of the lightest cold, the coronavirus test will not work. You need a blood test for antibodies to coronavirus. And that's why.

What is a coronavirus antibody test and how does it work

The PCR test allows you to "catch" the actual coronavirus, if it is in the body. But the test for antibodies (also known as enzyme-linked immunosorbent assay, ELISA) helps to see not the virus as such, but the immune response. What is the diagnostic accuracy of antibody tests for detecting infection with the COVID-19 virus? on his invasion.

Human immunity reacts to infection, including COVID-19, by producing and releasing specific proteins into the bloodstream, tuned to search for and destroy the virus. They are called antibodies.

Antibodies do not appear immediately. It takes time for the body to recognize an infection, isolate its "special features" and grow proteins aimed at destroying specific invaders. The antibody level rises gradually.

If there are enough antibodies to stop the multiplication of the virus and cleanse the body of it, the person recovers.

These antibodies, which directly fight infection, are called class M immunoglobulins (IgM). When the disease recedes, the place of IgM is taken by class G immunoglobulins (IgG). There are fewer of them, and they have a slightly different task: they keep a "portrait" of the defeated virus. If the organism encounters it again, the IgG proteins will recognize the intruder and start mass production of IgM “fighter” antibodies. Thus, the immune response is accelerated, and with the next contact with the pathogen, the immune system manages to defeat it before it multiplies.

The coronavirus antibody test detects both types of antibodies. And thus make a conclusion about whether a person was sick with COVID-19 and whether he has immunity to this disease.

How is a coronavirus antibody test done?

To test for antibodies to coronavirus, blood will be taken from your finger or from a vein. Next, the resulting drop is applied to a special test kit with indicator strips. Typically, it has three stripes:

• С (control) - control, it shows whether the test is working in principle;
• IgM - it fixes class M immunoglobulins;
• IgG - for the fixation of class G immunoglobulins.

But there may be other options. For example, there are tests without a control strip or tests that only detect one type of antibody. In addition, blood is sometimes tested in a laboratory setting.

How to decipher the test for antibodies to coronavirus

This is not difficult. For simplicity, let's take one of the popular test options.

The control strip (C) will be highlighted in any case, it does not say anything about the person's relationship with COVID-19. Indicative are only indicators that record the presence or absence of IgM and IgG antibodies. If Development and clinical application of a rapid IgM ‐ IgG combined antibody test for SARS ‐ CoV ‐ 2 infection diagnosis is found:

• IgM - it means that a person is sick with COVID-19 right now or has been ill recently;
• IgG - it means that a person was ill with COVID-19 some time ago, and he developed immunity;
• IgM and IgG mean both situations at the same time.

How accurate is the coronavirus antibody test?

Unfortunately, antibody tests can lie: give both false positive (that is, show that a person was sick, although in fact he did not have a coronavirus), and false negative results. This is due to many factors.

1. Analysis done too early

IgM antibodies, as studies show, in the case of SARS ‐ CoV ‐ 2, Development and clinical application of a rapid IgM ‐ IgG combined antibody test for SARS ‐ CoV ‐ 2 infection diagnosis appears no earlier than 3–6 days after infection. Sometimes later. IgG antibodies in sufficient quantities appear at least after at least 8 days.

This means that you may already be sick with COVID-19 with all the symptoms, but a test for antibodies to coronavirus will show a negative result.

Here's what they say about antibody testing. What is the diagnostic accuracy of antibody tests for COVID-19 infection? experts:

• A week after the onset of the first symptoms, the test can detect only 30% of people with COVID-19. In other cases, the disease remains unrecognized.
• After two weeks, the accuracy rises to 70%. But in 30% of people with coronavirus infection, the antibody test will still show a false negative result.
• After three weeks, the accuracy of the analysis is the highest - over 90%.

2. Analysis done too late

IgM antibodies disappear soon after recovery. How long after a disease in a person's blood IgG antibodies remain (that is, how long immunity to the causative agent COVID-19 lasts) is currently unknown Interim Guidelines for COVID-19 Antibody Testing.

It is possible that IgG will be recorded in someone even a year after the previous illness. And from someone - they will disappear in a month or two. That is, an antibody test done a couple of months after the illness may not show that the person was infected with a coronavirus infection at all.

3. Sometimes tests fail on their own

A technical failure may occur, and the test will detect the presence of antibodies to the coronavirus - although in fact the person is not sick or has some other infection. These false positives occur in 2-21%. What is the diagnostic accuracy of antibody tests for COVID-19 infection? cases.

The spread is related to the stage of the disease at which the test is made. The earlier the analysis is done, the less the risk of a false positive result.

4. A positive test result does not mean that you have immunity

Scientists today do not know what level of antibodies must be present in the blood in order to protect a particular person from coronavirus. Perhaps this level will be high enough for the test to catch. But it is completely inadequate to resist infection.

Why do coronavirus antibody tests if they are often inaccurate?

From the point of view of each individual, antibody testing may indeed seem useless. But Interim Guidelines for COVID-19 Antibody Testing plays an important role when it comes to society as a whole. Here are just some of the reasons for this.

1. Tests allow you to estimate the proportion of those who have recovered

This is important for assessing herd immunity. It is believed that herd immunity will begin to work only when 70% of the First and second waves of coronavirus of the population are ill with COVID-19.

2. Tests reveal how many people are suffering from coronavirus infection without symptoms

This is important for assessing the severity of the disease, its mortality and identification of groups at particular risk.

3. Doctors need tests

And representatives of other professions who are forced to constantly contact people (among whom there may be asymptomatic patients) - teachers, salesmen, taxi drivers. Information about which of the specialists has already been ill, that is, is most likely immune to COVID-19, will help to correctly distribute the workload. For example, it is safer to send a doctor who has already had an infection to the emergency room where patients with symptoms of coronavirus are admitted.

4. Tests are needed to clarify the diagnosis

PCR analysis also does not have sufficient accuracy and sometimes shows negative results several times in a row - although the patient has obvious symptoms of coronavirus infection. In this case, the antibody test can be used as an additional, screening diagnostic method.

5. Tests are needed to check the effectiveness of vaccines against coronavirus

The task of the vaccine is to form antibodies to COVID-19 (IgG) in a healthy person. In this case, when faced with a real infection, the immune system will immediately begin to destroy it, and the patient himself will transfer the coronavirus asymptomatically or easily.

You can only check if antibodies to COVID-19 have appeared after vaccination with a test.

Coronavirus. Number of infected:

243 050 862

in the world

8 131 164

in Russia View map