Page 193 - TBE_Book_V2_2019

Chapter 10: Diagnosis

Assays evaluating IgG antibodies are usually against these viruses, have occurred or not.
produced in a conventional 2-layer sandwich Therefore, a broad cross-reactivity against
format. Anti-TBEV IgG is broadly cross-reactive different flaviviruses or high IgG antibody
with other anti-flavivirus IgG antibodies. titers should raise the suspicion of a secondary
ELISAs for detecting IgG anti-TBEV antibodies immune response (Figure 2). Patients with TBE
display a high sensitivity (up to 99%), but only vaccination failure can often also display a
moderate specificity (40–80%) if sera from serologic pattern consistent with a secondary
patients or vaccinees exposed to other immune response.
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flaviviruses are tested. The specificity can be
up to 97%, however, when samples with no Avidity testing
history of exposure to other flaviviruses are
tested. IgG antibodies against TBEV are The avidity of an antibody is an artificial index
usually present at the onset of CNS that indicates the binding activity of an
symptoms, reach a maximum titer after about antibody to a specific antigen. The avidity of
6 weeks, and persist for years. The antibody an antibody usually increases with time after
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titers present after natural infections are infection and reaches its peak after weeks to
usually much higher than those that develop months. The avidity index may therefore help
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after vaccination. to differentiate recent and past infections. The
testing of avidity is performed by testing the
As with diagnostic tests for other flaviviruses, sera in parallel ELISAs with and without
different types of antigen have been washing with 8M urea. The avidity index is
investigated in ELISAs in order to increase the calculated as a percentage using the formula:
sensitivity and specificity of testing. The use of (optical density [OD] of IgG with urea / OD of
NS1 protein as the antigen to be detected IgG without urea) ×100. Sera with an avidity
shows some increase in specificity but a index <40% are of low avidity and indicate a
decrease in sensitivity. ELISAs based on NS1 recent infection, whereas an avidity index
do not detect anti-TBEV antibodies after >80% indicates an old infection. Avidity testing
vaccination, and therefore this format could is used in suspected West Nile virus infections
be capable of distinguishing between an as there is sometimes a persistent IgM that
infection-induced and vaccination-induced can confound interpretation of whether an
immune response, which might be a relevant infection is recent or not. In TBEV infections,
diagnostic question when CNS symptoms persistent IgM from a past infection is
occur after vaccination. uncommon and therefore avidity testing is not
routinely performed in cases of suspected
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Secondary antibody response type TBEV infection. In our laboratory, avidity
testing is used to differentiate passively
Pre-existing immunity due to previous transferred IgG antibodies from infection-
infection or vaccination with other flaviviruses induced antibodies, e.g. to exclude Guillain-
could modify the immune response to TBEV Barré syndrome in suspicious cases.
infection or TBE vaccination. In such cases, a Preliminary avidity testing of IgG in vaccinated
low IgM and high IgG antibody response can persons shows that high avidity IgG is only
usually be observed (the author’s personal produced after a complete basic vaccination
observation). In addition, a reactivity against (the author’s personal observation).
other flaviviruses (dengue virus, West Nile
virus, yellow fever virus, Japanese encephalitis
virus) can be observed independent of
whether these infections, or vaccinations

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