First, a disclaimer....I'm NOT an immunologist so what I'm about
to say is a little out of my field; if what I say is erroneous, please
feel free to flame me.
B-cell levels (or more easily measured gamma-globulin levels)
such as 'stormbri' is suggesting could be measured as an indicator for
chronic EBV infection vary widely from a number of causes, both due to
mild concurrent infections and due to genetic anomalies. In this latter
catagory Benign Monoclonal Gammopathy comes to mind in particular, as it
causes an asymptomatic increase in a specific B-cell line along with
large elevations in gamma-globulin levels. This particular 'disease' is
in fact quite common, with ~3% of people having it by middle age.
In short, B-cell levels are not a very useful diagnostic for the
suggested purpose both due to thier non-specificity as an infection
marker and the not insignificant probability of observing abnormally high
levels without a disease state.