Explanation

Varicella zoster antibodies (IgG): what is measured?

The varicella zoster virus (VZV) causes chickenpox and sometimes, later, shingles. After chickenpox or a vaccination, the immune system makes IgG antibodies (defense proteins) against the virus, which usually remain detectable for life. The test measures the amount of VZV IgG in the blood. Above the cut-off value of the test (100 or 150 IU/l, depending on the test), someone is considered protected against chickenpox.

What can it detect?

Protection against chickenpox after a past infection or vaccination
No detectable protection: a chance of chickenpox, with extra risks during a pregnancy

Result

What does the result mean?

Positive result

A positive result means that antibodies have been found: you have had chickenpox or have been vaccinated against it. According to the Dutch medical specialists' guideline (FMS) on varicella, you are then considered protected against chickenpox; chickenpox despite a positive result is rare. A pregnant woman with a positive result does not need to take extra measures after contact with someone with chickenpox or shingles. The result says nothing about the chance of shingles later, because shingles develops when your own virus, already present in the body, becomes active again.

Negative result

A negative result means that no or too few antibodies have been found; you may then still be able to get chickenpox. Because about 95 in 100 people in the Netherlands have antibodies, this is not common in adults, and the usual tests sometimes miss low amounts of antibodies: according to the FMS guideline, with a negative result there is a real chance that there is protection after all. A negative result is more common in people who grew up in a country where chickenpox is less common. During a pregnancy, chickenpox can be severe: about 5 in 100 pregnant women with chickenpox get pneumonia, and with an infection between weeks 13 and 20, about 2 in 100 children have a congenital abnormality.

After an abnormal result

With no antibodies and a wish to become pregnant, the GP can discuss vaccination against chickenpox; after the vaccination, the advice is not to become pregnant for 4 weeks. If a pregnant woman without detectable antibodies has had contact with someone with chickenpox or shingles, she can receive antibodies (immunoglobulin) within 10 days via the midwife, GP or hospital. Individual cases of chickenpox are not notifiable.

Reliability

How reliable is the test?

According to the Dutch medical specialists' guideline (FMS) on varicella, someone with detectable VZV IgG is considered protected and someone without detectable IgG is considered not protected. The tests widely used in the Netherlands are very specific (more than 98 percent), but miss some low antibody levels: a positive result therefore predicts protection very well, a negative result much less so. Your medical history is also reliable: anyone who knows for sure they have had chickenpox is almost always protected, while 43 to 75 percent of adults who think they have never had it do have antibodies. According to the LCI guideline (RIVM), IgM is not very reliable for showing a current infection.

What can affect the result?

The cut-off value differs per test (100 or 150 IU/l); a result just below the cut-off value can still fit protection.
After vaccination, the amount of antibodies can be lower than after having had chickenpox and sometimes falls below the cut-off value.
After a blood transfusion or administration of immunoglobulins, antibodies from donors can temporarily be measured as well.
With a weakened immune system, the amount of antibodies says less about protection.

Who it is for

Who is the test useful for?

Useful for

Useful for women who are planning a pregnancy or are pregnant and do not know for sure whether they have had chickenpox; the Dutch medical specialists' guideline (FMS) on varicella recommends discussing the test if your history is negative or uncertain. Also useful for people who work with children or vulnerable patients and have not definitely had chickenpox or grew up outside Western Europe, and before a treatment that strongly suppresses the immune system. Anyone who knows for sure they have had chickenpox, or in whose family a child had chickenpox, usually does not need the test.

How common is it?

In the Netherlands, about 95 in 100 people between 0 and 79 years of age have antibodies against the chickenpox virus; among 6-year-olds this is already more than 95 in 100. In 2022, GPs registered about 4 in 1000 people with chickenpox, mainly young children. Vaccination against chickenpox is not part of the National Immunization Program in the European Netherlands.

The LCI guideline (RIVM) on chickenpox, based on the Pienter study and NIVEL

Drawbacks and risks

A negative result in someone who is in fact protected, for example after vaccination or with antibodies just below the cut-off value, can cause unnecessary worry or an unnecessary vaccination.
A negative result during pregnancy can cause concern, while vaccination is then no longer possible.

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