*7.3.2. Serology*

In the routine clinical practice, demonstration of antibodies to TBEV in serum (and in some cases also in CSF) by enzyme‐linked immunosorbent assay (ELISA) is a standard microbiologic diagnostic approach with a high sensitivity and specificity [98, 99]. At the beginning of the meningoencephalitic phase, when patients are usually seen by their physicians and admitted to hospital, the large majority had specific serum IgM and IgG antibodies. In rare cases when only IgM antibodies to TBEV are found in the first serum sample, second sampling 1–2 weeks later reveals IgG seroconversion and enables a reliable diagnosis of (recent) TBEV infection. In CSF, IgM and IgG antibodies to TBEV appear several days later than in serum, but are detectable in almost all cases by day 10 [98, 100].

Although the interpretation of the results of serological testing is usually straightforward, there may be some obstacles which should be taken into account. TBEV IgM antibodies may be present in serum for several months (up to 10 months or even longer) after acute infection, whereas TBEV IgG antibodies persist for a whole life and mediate an immunity that prevents symptomatic reinfection [98, 101].

Thus, serum IgG antibodies to TBEV without the presence of specific IgM antibodies do not indicate a recent but previous (symptomatic or asymptomatic) TBEV infection or vaccination against TBE. On the other hand, specific TBE serum IgM antibodies, an indicator of a recent infection with TBEV, may be detectable for several months after acute TBEV infection (and also in some persons after the first two doses of primary immunization); their demonstration may result in incorrect interpretation if another CNS infection/disease developed within this time period [98, 101].

A further challenge is a close antigenic relationship between TBEV and other flaviviruses with cross-reactive antibodies induced by infections or vaccinations, and a consequent diagnostic difficulties in persons vaccinated against Japanese encephalitis or yellow fever and in travelers having acquired dengue, West Nile or other flavivirus infections [7]. Such problems in TBE serodiagnosis can be sorted out by the quantification of IgM antibodies. High IgM values (>500 arbitrary units) are indicative of a recent infection with TBEV, whereas lower IgM levels may require the analysis of a follow-up sample (that enables the assessment of antibody dynamics), and/or a specific neutralization assay, to rule out cross‐reactive IgM antibodies and prolonged persistence of IgM antibodies after infection or vaccination [102].

Knowledge in the understanding of TBE serology is required also in patients with meningitis or meningoencephalitis or who had been previously vaccinated against TBE. Serological response in patients with TBE vaccination breakthroughs is as a rule distinct from the response in patients who had not been vaccinated; unawareness of the pattern may result in fail to notice vaccination breakthrough cases. Serologic response in these patients is characterized by a delayed development of specific IgM response (during the initial days of the meningoencephalitic phase of TBE, specific IgM antibodies may not be detectable) associated with a high and rapidly increasing levels of specific serum IgG antibodies [63, 64, 67]. For a reliable diagnosis of TBE in persons previously vaccinated against TBE, demonstration of intrathecal production of TBEV antibodies is needed [45].
