Overview on Testing for HHV-6 infection
Nearly 100% of us are infected with HHV-6B by early childhood. Little is known about the prevalence of HHV-6A, which is believed to be acquired later in childhood. Both HHV-6 viruses are highly cell associated and can be detected in plasma only briefly during the initial infection or acute reactivation. Most reactivation occurs in transplant patients taking immunosuppressant drugs or individuals with immune deficiencies. Detection of HHV-6 DNA in plasma generally means the patient has an active infection.
It is important to note that a negative finding in the plasma does not rule out a localized active infection in an organ (e.g. uterus, brain, thyroid, liver). Persistent HHV-6 infections have been found in the liver, brain, lungs, heart tissue and uterus, with no trace of HHV-6 DNA in the plasma. Quantitative testing on blood and tissues is preferred because it can differentiate between the very low levels occasionally found in healthy controls and high levels found in diseased tissues.
Over 95% of us have antibodies to HHV-6 A/B. The relevant question when testing for HHV-6 antibodies is not whether you have them, but rather, are they unusually elevated on an IFA test? Unfortunately, many laboratories just use the less expensive ELISA test that was only designed to answer the question of whether the antibodies exist (so fairly useless for HHV-6 testing). At the time of this publication we are aware of only three laboratories in the USA that offer antibody testing by IFA methodology – Quest Diagnostics, Eurofins-Viracor and ARUP Laboratories.
Physicians and patients need to be aware that there is a rare inherited form of infection known as “chromosomally integrated HHV-6” (ciHHV-6). Individuals with this condition, which occurs in 1% of the population, have one copy of the viral genome integrated into the chromosome of every nucleated cell, and therefore test positive by PCR DNA, even when asymptomatic. (See What is ciHHV-6?)
| 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 |
If the titer is highly elevated relative to healthy controls, it means that the patient may have had a recent infection or has a current chronic/smoldering infection. Quest Diagnostics, Eurofins-Viracor and ARUP Laboratories are three of the only known laboratories in the USA to perform testing by IFA methodology. Their labs have 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 aa active infection. Chronic infections in various tissues can persist 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 acute infection (or the individual has ciHHV-6). However, the absence of HHV-6 DNA in the plasma/serum does not mean that there isn’t a low-level persistent infection in the tissues (e.g. heart, thyroid, brain). | 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) |
| 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. Usually this level is not detectable in a commercial lab. |
| 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. Almost all healthy individuals have low levels of latent HHV-6B in the blood. On a sensitive nested PCR tests, at least a third of the normal population should test positive for HHV-6 latent DNA. | This test may be useful for determining if you have HHV-6A or HHV-6B but can’t tell you if the virus is active. |
| Immunohistochemistry
Staining of tissue sections from biopsies, mounted on glass slides. |
YES. This test can tell you if the proteins expressed were from replicating virus. Only Coppe Labs and IKDT perform this service commercially. | This technique can also determine whether HHV-6A or HHV-6B is active. Analysis usually done on formalin fixed, paraffin embedded material. |
| Tissue Biopsy – qualitative | NO. This test cannot tell you if the virus is active. Eurofins-Viracor offers a qualitative PCR test on tissues (liver, uterine, kidney, GI tract). Code: 6506 Coppe Labs can perform immunohistochemistry from slides. | Depending on the lab, the virus can be typed. Specimens should be sent frozen overnight, with no liquid added. Slides should be 4 microns and unstained. |
| Tissue Biopsy – quantitative | YES. This test can differentiate between low-level latent virus and active virus with high copy numbers.
Eurofins-Viracor (Code 6505) and can test tissues sent frozen in a sterile container. |
Viracor requires 5 mg of material. Coppe can test 1 mg of material.
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| ddPCR or Digital Droplet PCR to confirm ciHHV-6 status | This ddPCR test is done on whole blood and was introduced in 2013 by University of Washington specifically to identify ciHHV-6. It cannot identify active infection. | Patients should request this test when physicians want to confirm suspected ciHHV6. |
| Menstrual Fluid – Quantitative | A positive result indicates the presence of HHV-6A DNA in the endometrium. The issue of Latent vs Active is less important because detection of HHV-6 DNA means that virus is present and can reactivate at any time in the menstrual cycle even if detected at a low level. | Fertility Phoenix offers this testing via Covee.
1-2ml menstrual fluid sample is collected at home then the sample is mailed back for testing. |