Explanation

Varicella zoster virus IgM: what is measured?

The varicella zoster virus (VZV) causes chickenpox and can become active again years later as shingles. In a first infection, the immune system first makes IgM antibodies (defense proteins), which disappear again after a few weeks to months; after that, IgG antibodies remain present for life. IgM is therefore used as a possible sign of a recent infection. For the question of whether someone is protected, IgG is measured, not IgM.

What can it detect?

A possibly recent first infection with the chickenpox virus

Result

What does the result mean?

Positive result

IgM antibodies have been found. This may fit a recent infection with the chickenpox virus, but the result on its own is not very reliable. IgM can also appear when the virus becomes active again, for example in shingles, and false positive results occur, among other things due to an infection with another virus. In chickenpox and shingles, the appearance of the skin usually says more than this result; if in doubt, a PCR on fluid from a blister detects the virus itself.

Negative result

No IgM antibodies have been found. This makes a recent first infection less likely, but does not rule it out: very early in the illness IgM may still be absent, and not everyone makes detectable IgM. A negative IgM says nothing about protection; IgG is the test for that. With blisters that look like chickenpox or shingles, the appearance of the skin and, if needed, a PCR remain decisive.

After an abnormal result

If the diagnosis is in doubt, the result is assessed together with IgG and a PCR on fluid from a blister can follow. A pregnant woman without detectable protection who had contact with chickenpox or shingles can be given antibodies within 10 days via the midwife, GP or hospital. Individual cases of chickenpox and shingles are not notifiable.

Reliability

How reliable is the test?

According to the LCI guideline (RIVM) on chickenpox, IgM is not very reliable for showing a current infection. The appearance of the skin is usually characteristic; a PCR on blister fluid detects the virus itself and is more reliable than IgM, also in people with a weakened immune system. For the question of whether someone is protected against chickenpox, for example around a pregnancy, IgG is measured. A stand-alone IgM result in someone without symptoms is hard to interpret.

What can affect the result?

IgM can also become positive when the virus becomes active again, such as in shingles, and then says nothing about a first infection.
False positive results occur, among other things with an infection with another virus.
Very early in the illness IgM can still be negative, and not everyone makes detectable IgM.
Test methods and cut-off values differ between laboratories.

Who it is for

Who is the test useful for?

Useful for

The result is most meaningful with symptoms or skin abnormalities that fit chickenpox where the picture is not clear, and then usually together with IgG and a PCR. For the question of whether someone is protected, for example when planning a pregnancy, during a pregnancy or when working with vulnerable people, IgG is the test and not IgM. In people without symptoms, a stand-alone IgM result is hard to interpret.

How common is it?

In the Netherlands, about 95 in 100 people have antibodies against the chickenpox virus; most people have chickenpox as a child. In 2022, GPs registered about 4 in 1000 people with chickenpox, mainly young children. Shingles develops when your own virus becomes active again later and occurs mainly at older age and with a weakened immune system.

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

Drawbacks and risks

A false positive IgM can wrongly attribute symptoms to the chickenpox virus, so that another cause is found later.
An IgM result is easily confused with a result about protection; only IgG says something about immunity.

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