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Viruses are difficult to detect when they are chronic. It
is very difficult to detect chronic infections of HHV-6 and
EBV. One reason is that 90% of us have been exposed to
these viruses by early childhood and the viruses remain latent in our cells.
Therefore, in order to differentiate active from latent infection, tests
must be done in the serum (the clear liquid portion of the blood)
on the theory
that in actively replicating virus, viral particles will leave
the cell and spill
into the plasma. The problem is that, unlike most viruses, HHV-6 and EBV
are very “cell associated” which means that the
virus rarely leaves the cell. In fact, with HHV-6, transmission
is largely cell-to-cell or directly
through
the cell walls. The net result is that it becomes very difficult to detect
active infection because there are very few viral particles in the serum.
The other reason chronic infections are difficult to detect is that these viruses
(especially HHV-6A) migrate to the central nervous system and other organs and
away from the blood. For example, HHV-6A has been found to persist in the spinal
fluid long after it has disappeared from the plasma.
This means that indirect signs of the virus (antibodies to the viral proteins)
become more important and antibody assays are more sensitive than molecular assays
(PCR) for differentiating latent from active chronic infections.
PCR tests for HHV-6 & EBV. These tests are currently
not useful for detecting most cases of Virus Induced CNS Dysfunction. While
PCR tests can easily detect
primary HHV-6B roseola infections and acute mononucleosis, they are not sensitive
enough to detect chronic, persistent central nervous system infections. A
negative PCR test done in serum or plasma is meaningless because the test
itself is hopelessly
insensitive. At the same time, a positive PCR test on whole blood is also
meaningless until a threshold is established that can distinguish between
healthy controls
and patients. Since 90% of us have latent virus in the cells, a whole blood
test will detect latent virus in the cells of healthy controls as well as
patients.
Many healthy individuals, especially young adults, have detectable levels
of latent virus in their whole blood.
Only a positive test in the plasma or serum is a significant result, although
extremely rare. Not one of Montoya’s patients who benefited from antiviral
treatment (and were subsequently determined to have active infections) was
positive by PCR. A high positive result for HHV-6 may mean that the patient
has chromosomally
integrated HHV-6. This is a rare form of HHV-6 that is inherited and was
found in 0.2% of the population in one large study in Japan.
IgG Antibodies.
These antibody levels indicate that a person
has had an infection at some point in the past; at high levels they can suggest
(but not prove) that
the virus is active. Much more research is needed on this question but in
one study of HHV-6 in CFS patients, 89% of the patients with HHV-6 IgG antibody
titer of
1:320 and above were found to have active infections by culture (Wagner 1996).
Results vary by laboratory, but at most commercial labs, healthy adults have
titers of 1:40 to 1:160. Sometimes a different ELISA assay is used and the
results are reported as an index. These index scores can also vary by laboratory
depending
on the manufacturer of the kit. Young adults may have unusually high antibodies
due to reactivation during mononucleosis.
IgM Antibodies.
This is not a very
useful test. Although it is worth testing once, this result is rarely positive
in patients
with chronic viral infections.
Typically IgM titers persist only for a few weeks after the primary infection.
Just because the IgM is normal does not mean that you don’t have an active
infection; many clinicians are not aware of this fact.
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Antibody testing for EBV is more complicated. There are three different types
of EBV assays offered at commercial laboratories. These antigens are the viral
capsid antigen (VCA), the early antigen (EA), and the EBV nuclear antigen (EBNA).
In addition, differentiation of immunoglobulin G and M subclasses to the VCA
can often be helpful for confirmation. The optimal combination of EBV serologic
testing consists of the antibody titers to all four markers: IgM and IgG to
the
VCA, antibody to the EA
Viral Capsid Antibodies (VCA).
IgG antibodies to the viral
capsid antigen develop 2 to 4 weeks after onset of the initial and then persist
for years, at
gradually declining levels. Drs. Montoya and Kogelnik at Stanford have found
that patients with elevated antibodies to VCA IgG and HHV-6 antibodies respond
to antivirals, and the VCA titers dropped significantly with treatment, suggesting
that elevated VCA titers represent active infection. This test is not definitive
since many healthy adults have relatively high antibodies as well. A high titer
in a 45 year old is far more significant than the same titer in a 20 year old.
IgM antibody tests are not very useful since they are typically found only in
primary or initial
infections, not chronic or reactivated infection.
Early Antigen (EA) Antibodies. IgG.
The early antigen (EA) antibody appears
during active replication phase and generally falls to low levels after 3 to
6 months. Most healthy adults have very low levels of early antigen; higher
levels however can be a sensitive marker of active infection in chronic disease.
There
is consensus that a titer of 1:640 is indicative of active disease, and that
a titer of <1: 80 suggests there is a far smaller chance of an active
infection. Results vary by laboratory and much more testing needs to be done
to establish
cutoff values, but the early antigen IgG antibody test remains our best clue
for active infection. Again, IgM elevations are typically found only in
a primary infection, not a reactivated infection, so the IgM test is usually
not very meaningful for chronic
infections.
Epstein-Barr Nuclear Antigen Antibodies (EBNA).
Antibody to EBNA
is not seen in the acute phase, but slowly appears 2 to 4 months after onset,
and
persists
for life. Most physicians use of this test is to determine if an EBV infection
is the initial infection or a secondary/ reactivated infection. A low EBNA
in the presence of a high VCA titer suggests an immune deficiency. For reactivated
infections, only the absolute level of the IgG titer usually significant.
There are several commercial laboratories that will test for HHV-6 and EBV.
However, not all labs use the same metric. For example, Focus Diagnostics
and Specialty
Laboratories use an IFA method with results reported as titers (1:80, 1:160
etc). Other labs such as Mayo Clinic, Quest and Labcorp use the ELISA method
and report
with an index. There is no standard data publicly available that would allow
patients to compare IFA and ELISA scores. The Stanford group currently conducting
a trial of Valcyte for these viruses uses Focus Diagnostics as their reference
laboratory because they are familiar with their scale and can interpret the
results.
The best way to interpret the results from your laboratory is to ask your
doctor to find out median and range values at the laboratory for controls
or blood donors. If your result is in the top quartile you are more likely
to have an infection, but there is not way to know for certain. Patients
who are immunosuppressed and have a low IgG may show up with low antibody
levels in spite of active disease.
If the IgG is low normal or below normal, then the HHV-6 and EBV antibody
test results may also be suppressed. Similarly, some patients with
very high
IgG may have high EBV and HHV-6 antibody levels that do not indicate active
disease. These considerations need to be evaluated by a knowledgeable physician. |
