if you have Primary Immune Deficiency or Secondary then your body doesn't have the antibodies for viruses in high enough concentrations or at all for a standard titer range test to pick them up. This would be why newer doctors seem to recommend DNA based or PCR based tests for accuracy, assuming the microbe is in the blood.
I am not sure if those with IgG subclass deficiency would equate to lower antibody levels in a blood test
I was deficient in both IgG subclasses 1 and 3 and had several doctors miss my EBV because they insisted on doing antibody tests. When my specialist finally did a PCR test, which was positive, he explained that I wasn't making antibodies well enough to have a positive EBV result by that method.
The reason they do antibody testing for Epstein-Barr is to stage the illness, e.g. Do you have a first time acute infection, a reactivated infection, or a past infection? This is a little silly in our case, because most of us have had the acute EBV infection and that's long gone, so we're trying to figure out if there's a reactivated infection. A PCR test will do this, which is a lot better than mucking around with antibodies that we may not be making. Then, as Amy Proal has shared, sometimes the virus is hiding out in the tissues and not in the blood, so repeating a PCR test a couple more times over two or three months would be wise if one wants to actually catch this virus.
