The actual diagnosis of TBE must be established in the laboratory because of the non-specific clinical features it presents. The laboratory results have no influence on the therapy of TBE, and mainly serve to differentiate a TBE virus infection from other causes of meningoencephalitis, which may require special treatment.
The method of choice is the demonstration of specific IgM and IgG serum antibodies by enzyme-linked immuno-sorbent assay (ELISA). As the symptoms affecting the CNS are not usually observed until two to four weeks after the tick bite, the antibody test is nearly invariably positive at the time of admission to hospital. Soon after infection IgM antibodies are more specific, while later, IgG antibodies are more reactive. A recent infection can be established by the qualitative determination of IgM. Specific IgG antibodies and rheumatoid factors do not interfere with the test. However, in cases of other flavivirus contacts (e.g. vaccinations against yellow fever or Japanese encephalitis; dengue virus infections) the performance of a neutralization assay (e.g. RFFIT, rapid fluorescent focus inhibition test) is necessary for assessing immunity due to the interference of flavivirus cross-reactive antibodies in ELISA and hemagglutination inhibition test.
The high cross-reactivity rate of yellow fever and TBE antibody-positive sera in dengue virus antibody assays should be taken into account in the interpretation of laboratory tests for the diagnosis of flavivirus infections, and when undertaking seroepidemiological surveys.Neutralization tests – like RFFIT – require handling infectious virus, which makes the test cumbersome, costly and only available in highly specialized virus laboratories. IgG antibodies are detected by qualitative methods, e.g. establish the prevalence of TBE in the population or in a group of patients, or follow seroconversion after active immunization and control of humoral immune status.
In the past, antibody identification relied on four tests: hemagglutination inhibition, complement fixation, plaque reduction neutralization test, and the indirect fluorescent antibody (IFA) test. Positive identification using these IgM and IgG based assays requires a four-fold increase in titer between acute and convalescent serum samples. The sensitivity and specificity of new assays, e.g. IIFT (indirect immunofluorescence test), ELISA, and immunoblot test systems are considerably higher than that of tests used in the past, and cross-reactivity with related flaviviruses is significantly reduced.
In the viremic phase of the initial stage of the disease before seroconversion, TBE virus can also be identified by reverse-transcriptase polymerase chain reaction (RT-PCR), by electron-microscopy, or by cultivation. In fatal cases, the virus can be isolated or detected by RT-PCR from the brain and other organs. Electron-microscopy and cultivation are not suitable as a routine diagnostic tool. In contrast to many other flaviviruses, the PCR method is not very useful for the laboratory diagnosis of TBE in clinical practice.
The overall prevalence of TBE can be established serologically, since the infection leads to immunity for life and the presence of antibodies to the TBE virus in the patient’s blood.