Explanation

HSV-1/2 IgM (herpes simplex): what is measured?

IgM antibodies are the antibodies the body makes first during a new infection; IgG antibodies come later and stay. This test looks for IgM against herpes simplex virus type 1 and 2 combined, without distinguishing between the types. In theory, IgM points to a recent first infection. In practice, IgM against herpes is difficult to interpret, because it is not always detectable in a first infection and false-positive results occur.

What can it detect?

Possibly a recent first infection with herpes simplex type 1 or 2

Result

What does the result mean?

Positive result

A positive IgM result may fit a recent first infection with herpes simplex, but on its own it is not proof. IgM tests give a false-positive result more often than IgG tests, for example due to a general immune reaction or cross-reaction. The result also does not say which type it is or whether the infection is genital or around the mouth. Only together with the IgG result, any symptoms and, if needed, a repeat measurement can it be assessed whether it is a new infection.

Negative result

A negative IgM result makes a recent first infection less likely, but does not rule it out. In the first weeks after infection, IgM is often not yet detectable. An older herpes infection cannot be seen with IgM; IgG is needed for that.

After an abnormal result

With a positive IgM result without symptoms, usually no treatment follows; after a few weeks, a doctor can have the antibodies measured again to see whether IgG has developed. With blisters or sores, the GP or the GGD (municipal public health service) does a PCR test, and pregnant women with a suspected first genital herpes infection are referred to a gynecologist. Herpes is not notifiable.

Reliability

How reliable is the test?

IgM against herpes is not always detectable in a first infection, and false-positive results occur. A PCR test on blister fluid detects the virus itself and therefore says more for diagnosing herpes; as a blood test, a type-specific IgG test says more than IgM. An IgM result can therefore only be interpreted cautiously, together with IgG and the clinical picture.

What can affect the result?

False-positive IgM results occur, so a positive result without symptoms says little.
The test does not distinguish between HSV-1 and HSV-2 and does not say where the infection is.
Shortly after infection, IgM is often not yet detectable; a negative result does not rule out a recent infection.

Who it is for

Who is the test useful for?

Useful for

In people without symptoms, a positive result says little, and with blisters or sores a PCR test says more. The test is not suitable for checking whether you caught herpes after a risk contact. The result says the most together with IgG, when a doctor wants to know whether symptoms fit a first infection.

How common is it?

How often a recent first herpes simplex infection occurs is not well known, because most infections cause no symptoms. In a national population study (2006 to 2007), 42.7 percent of inhabitants had antibodies against HSV-1 and 6 percent against HSV-2. In 2023, GPs made the diagnosis of genital herpes an estimated 28,600 times.

LCI guideline (RIVM) on herpes simplex virus infections (Woestenberg 2016); RIVM

Drawbacks and risks

A false-positive IgM result can cause unnecessary worry and unnecessary follow-up tests.
A negative result can give false reassurance, because IgM may not yet be detectable in a recent infection.

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