Nearly 100% of us are infected with HHV-6 by early childhood. This initial infection has caused our bodies to develop HHV-6 antibodies to fight against future HHV-6 viral infection or reactivation. In addition, at least 30% of us have small but consistently detectable levels of inactive (latent) HHV-6 virus that persist harmlessly in the blood into adulthood—many times never reactivating or becoming active over the course of a lifetime. Therefore, the relevant question when testing for HHV-6 infection is not
whether you have HHV-6 virus or antibodies present in your blood, but rather is the virus
active or
latent? Unfortunately, this is not an easy question to answer. Two signs of an active infection are (1)
viral DNA in your plasma or serum (as opposed to blood), and (2) unusually elevated antibody titers, on an IFA test. A PCR test on blood identifies both latent and active infections, so it is not helpful in answering this question unless it is a quantitative test.
TEST
Can it differentiate active from latent infection?
Comments
ELISA IgG test
Example: “positive >1.0″ No. These results are intended to give only “yes/no” answers to whether you are exposed to the virus in the past. An ELISA> 5 in an adult MIGHT be a clue of an active infection, but only the antibody tests done by IFA can tell you with precision how elevated the antibodies are.
IFA IgG test
Example: Titer 1:640 Sometimes! If the titer is highly elevated, it means that the patient has had a recent infection or currently has a chronic infection. Titers vary by laboratory. Focus Diagnostics (Quest) has a median antibody titer between 1:80 and 1:160 for controls. Other labs have much lower control titers. If the patient has an immune deficiency with low total IgG, then the antibody titer will not be elevated. If HHV-6 is the only antibody titer out of five viruses to be elevated above average, then this indicates possible HHV-6 infection.
IgM test
(ELISA or IFA) Yes. IgM only appears during an active infection or for 2-3 months after an active infection. The absence of an IgM antibody does not mean you do not have a active infection. Low level infections can perist with no evidence of IgM.
PCR DNA test on plasma or serum
(qualitative or quantitative) Yes. HHV-6 is never found in plasma or serum unless there is an active infection. However, the absence of HHV-6 DNA in the plasma/serum does not mean that there is no active infection. HHV-6 does not circulate in the plasma/serum except during the initial infection and transiently during an acute infection. Any positive test result should be repeated with a quantitative test. Also, a whole blood test should be ordered to rule out chromosomally integrated HHV-6 which occurs in 1% of the population. (See CIHHV-6) Someone who inherits HHV-6 in the chromosome will always be positive in the serum, whether the HHV-6 is active or not.
Quantitative PCR DNA test on whole blood
Example: 1200 copies/ml Yes. If the viral load is >200 copies per ml or 20 copies per microgram of DNA then this is an active infection. Healthy persons will have very low viral loads, typically less than 20 copies/ml in the whole blood.
Qualitative PCR DNA test on whole blood
Example: “Positive” with no numerical value given No. This test is useless for differentiating active from latent infection, and should be avoided. Most healthy individuals have small levels of latent HHV-6B in the blood. This test can be useful for determining if you have HHV-6A or HHV-6B Finding HHV-6A DNA is unusual and follow up testing might be worthwhile.
While both LabCorp and Quest laboratories offer a basic qualitative ELISA test for HHV-6, additional testing is necessary to determine a proper diagnosis of HHV-6 infection for the reasons listed above. FOCUS Diagnostics, a division of Quest Laboratories, offers many of the above tests that may help to properly identify an HHV-6 infection. Patients may have blood drawn at any Quest laboratory, and request that these specific testing inquiries be forwarded to FOCUS Diagnostics to ensure proper results.