TBE IgM (Tick-borne Encephalitis): Guide

What are tick-borne encephalitis IgM antibodies

Tick-borne encephalitis (TBE) IgM antibodies are early immune system proteins that appear within the first days or weeks after infection with the TBE virus. IgM antibodies indicate a possible recent contact with the virus. They are typically detected earlier than IgG, and their concentration decreases after several months.

The test is performed from venous blood using ELISA or chemiluminescence methods. The result is measured in VIEU/mL or U/mL units, and the positive result threshold depends on the laboratory test kit used.

Reference ranges

As with IgG, the TBE IgM test is qualitative — the result is classified as negative, equivocal, or positive. Each laboratory specifies its own cutoff values based on the kit used. There are no universal numerical norms.

When might the result be positive

• Early phase of TBE infection — IgM appears within 1–2 weeks of infection.
• Recently resolved infection — IgM can persist for several months after the illness.
• In rare cases — non-specific reaction or cross-reaction with other flaviviruses.
• In some cases, IgM may be detected after vaccination, although this is uncommon.

When might the result be negative

• The person was not infected with the TBE virus.
• The test was performed too early — IgM antibodies may not be detectable in the very first days of infection.
• Too much time has passed since the infection — IgM concentration has already declined.
• Immunosuppression — diminished immune function may lead to a weaker antibody response.

What to do with the results

A positive IgM result, especially in combination with clinical symptoms of TBE (fever, headache, meningeal signs), strongly supports the diagnosis of an active or recently resolved infection.

The doctor typically evaluates IgM together with IgG results:

• IgM positive, IgG negative — possible early infection.
• Both positive — infection is ongoing or recently resolved.
• IgM negative, IgG positive — past infection or vaccination.

In equivocal cases, repeating the test after 2–3 weeks is recommended.

Monitoring over time

IgM antibodies typically disappear within 2–6 months after infection. If treatment or monitoring is being conducted, the doctor may order repeat tests to assess dynamics. Long-term protection is evaluated based on IgG levels.

Questions for your doctor

1. Does my IgM result indicate an active infection, or could it be from an earlier time? 2. Should an IgG test be performed to get the full picture? 3. Should I have a repeat test after a few weeks? 4. What additional tests are needed if TBE infection is suspected?