Explanation

SARS-CoV-2 spike IgG: what is measured?

SARS-CoV-2 is the coronavirus that causes COVID-19. The spike protein sits on the outside of the virus; the virus uses it to enter cells. After an infection and after a COVID vaccination, the immune system makes IgG antibodies against this protein, because the COVID vaccines used in the Netherlands teach the body to recognize exactly this protein. This test measures how many of those antibodies are in the blood and gives the result as a number; it therefore cannot tell vaccination and infection apart.

What can it detect?

Antibodies after a COVID vaccination
A past infection with the coronavirus, even if it caused no symptoms
With a weakened immune system: whether the body has made antibodies after vaccination

Result

What does the result mean?

Positive result

A positive result means that antibodies against the spike protein were found in your blood. This fits a past COVID vaccination, a past infection, or both; the test cannot tell them apart. The laboratory also gives a number: the higher it is, the more antibodies. According to the LCI guideline (RIVM), antibodies cannot tell you anything about protection against COVID or about being contagious, and there is no level above which you are protected. So a high value does not mean that you can no longer get COVID.

Negative result

A negative result means that no antibodies against the spike protein were found, or too few to measure. After an infection, antibodies usually only become measurable 10 to 14 days after the start of symptoms, and after a vaccination they rise during the first two weeks. After a mild infection, at an older age and with a weakened immune system, not everyone makes measurable antibodies, and the amount falls in the months after a vaccination or infection. A negative result does not mean that you have no protection at all: part of the immune response, such as memory cells, is not measured by this test.

After an abnormal result

A result of this test usually does not lead to further testing or treatment. In someone with a weakened immune system who has no antibodies after vaccination, the treating doctor can assess whether extra protection is useful. COVID-19 has no longer been notifiable since 1 July 2023.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM), antibodies can show whether someone has had COVID or has been vaccinated, and whether someone is able to make antibodies, but they give no certainty about protection, being contagious, or transmission. After vaccination the test is almost always positive: in the PIENTER-Corona study by the RIVM (Dutch National Institute for Public Health and the Environment), 99 to 100 percent of adults aged 18 to 64 had antibodies after the primary vaccination with most vaccines. The number can only be compared well with an earlier result from the same laboratory using the same test.

What can affect the result?

Shortly after an infection or vaccination, up to about two weeks, there are often still few measurable antibodies.
The amount of antibodies falls in the months after a vaccination or infection, faster in older people than in younger people.
With a weakened immune system, due to an illness or to medicines that suppress the immune system, antibodies are made less often or in smaller amounts.
Numbers from different laboratories or tests cannot simply be compared with each other.

Who it is for

Who is the test useful for?

Useful for

The result says the most in people with a weakened immune system, due to an illness or medicines, when there is doubt whether the body has made antibodies after vaccination. The result does not say whether you are protected and is therefore not a measure of whether a new COVID vaccination is needed. This test is not suitable for showing an infection that is present now; that requires a test for the virus itself. If you want to know whether you have had COVID, separately from vaccination, the test for nucleocapsid antibodies is more useful.

How common is it?

In the PIENTER-Corona study by the RIVM (Dutch National Institute for Public Health and the Environment), about 95 percent of the Dutch population had antibodies against the coronavirus from spring 2022 onward, through vaccination, infection, or both. In autumn 2024 this was almost 100 percent. A positive result is therefore the expected outcome in the Netherlands.

RIVM, PIENTER-Corona study (2020 to 2024)

Drawbacks and risks

A high value can wrongly give the impression that you are protected against COVID.
A low or negative value can cause unnecessary worry, while part of the immune response is not measured by this test.

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