Explanation test results IgG rapid test

Please read about interpreting results of the IgG/IgM COVID-19 rapid test here.

What does  COVID-19 rapid test measure?

The test measures Immunogobulin M and Immunoglobulin G (IgM and IgG) against Sars-Cov2 (causing COVID-19). Your immune system produces antibodies to fight the virus. IgM is the fast reacting antibody, presumed to raise from 4 days to 2 weeks after the infection (this is still not very clear, based upon knowledge about other viral diseases). IgG is the longer acting antibody, levels raising 2-4 weeks after infection.

This means this test is only useful after 3-4 weeks after cure of a possible infection.

The test does NOT measure a current infection, this is done by a nose/throat swab.

So: test will tell you if your immune system has been in contact with the Sars-Cov2 virus in the past.

 

Is the test reliable?

At this moment no single test exists that is 100% reliable. Also the test you just did is still under investigation, the sensitivity seems to be around 90%, the specificity between 96-98%. This is for IgG, IgM is slightly less reliable.

This means that, if you had the infection, the test will be positive in 9 out of 10 cases , or negative in 1 of 10 cases (sensitivity) meaning it misses 10% of the infections.

Specificity of 96% means that if the test is negative, there is a 4% chance that the result is false positive: you did not have the disease, but the test indicates you had it.

 

What are the problems with the tests and the results?

1: Antibody levels are peaking only after 3-4 weeks.  This is why we advise to do the test 4 weeks after the last symptoms of an infection are gone, to make the chance that antibodies can be detected, higher. The test does not show a current infection, only if your immune system has been trying to clear a Sars-Cov2  infection in the past.

2: If you had only slight symptoms 3-4 weeks ago,  there might not be enough antibodies that can be detected, so: the more severe ill you have been, the more reliable the test becomes. You could have  gone through a slight form of COVID-19, but you did not make enough antibodies: The test can be negative, while there has been an infection. You might still be susceptible to a (new) infection.

4: The test can be positive, while there has been no infection. 

5: It is not clear if having antibodies will protect against future infections, though in theory IgG should protect you against a recurring infection during this COVID-19 season.



What would results tell you.

  • 1: The test was positive: There is a high probability that you had COVID-19, certainly if you had severe symptoms more than 3-4 weeks ago (fever, breathing problems, coughing, gastrointestinal problems). You are probably safe not to catch COVID-19 again this season (but we don’t know the duration of this season). You will probably not be a future source of spreading the virus. In 4 out of 100 cases however, you did not have the disease and are still at risk of catching COVID-19 or passing the virus on to vulnerable people. This can be very dangerous if you are visiting/helping/seeing vulnerable people. This leads to false security and is the biggest pitfall of a positive result.

 

  • 2: The test was negative: There is a high probability that you did not have COVID-19 but only if you had clear symptoms (fever, cough etc) and if you had these symptoms AT LEAST 3-4 weeks ago. Testing too early leads to more negative results. If you had slight or no symptoms 3-4 weeks previous to the test, a negative result could mean two things: you did indeed not have COVID-19, or your symptoms were too light so too little antibodies were made: the test can not detect them.

 

Concluding:

If a test is positive, there is a high probability that you had the disease, but you HAVE TO REALIZE that there is still a 4% chance that you are still at risk to develop (and spread) the Sars-Cov2 virus.

You should not use a positive test result to reassure yourself that you can not pass on the virus to other people, this is especially important if you take care of/visit elderly, or people with a impaired immunity (ie because of chemotherapy etc.). If the test were false positive, you did not have the disease and you are still a possible source of spreading the virus.

 Social distancing and good hygiene are still needed, regardless of the test result!

Share: