Detection of Active HHV-6 Infections

Wisconsin Viral Research Group, Ltd.

About HHV-6: Detection

How are active HHV-6 infections detected?

There are currently three basic methods for detecting the presence of HHV-6 in samples of human tissues and body fluids such as blood and bone marrow. Only virus isolation can detect active, replicating virus.

Serology

Serologic testing detects the presence of antibodies in serum. The antibodies, or immunoglobulins, tested for fall into two categories: IgG and IgM. These tests may give a qualitative (positive or negative) result or a semi-quantitative numeric result in the form of a titer, for example 1:120. A titer is a relative measurement of the amount of antibody in the sample, but in no way indicates a high or low positive when performed on a single sample. Titer results merely indicate that a sample is positive if the titer is greater than a predetermined cutoff titer (e.g. 1:10) or negative if the titer is less than this cutoff value. A positive result for IgG indicates a previous infection with HHV-6. Greater than 95% of the population is positive for IgG antibodies. This result has no correlation to active HHV-6 infections. A positive result in a serologic test for IgM indicates that the patient has recently produced a new population of antibodies against HHV-6. Testing for IgM antibodies poorly correlates with an active infection and normal, healthy individuals can test positive for HHV-6 IgM.

Polymerase Chain Reaction (PCR)

PCR, i.e. the amplification of specific nucleic acid sequences, using samples containing cells has likewise proven unreliable for the detection of active HHV-6. While these assays are capable of detecting viral DNA, the presence of viral DNA does not indicate an active infection. This style of assay also suffers from a high false negative rate due to inhibitors present in patient samples.

Virus Isolation

Virus isolation is considered the gold standard for identification of active viral infections in blood and tissues. There are currently three methods of virus isolation.

Immunohistochemical staining of tissue samples
Polymerase-chain-reaction (PCR) assays of cell-free samples
Cell culture

Immunohistochemical staining of tissue samples can detect active infection. This method requires a biopsy or autopsy sample, which limits its usefulness. PCR of cell-free samples, such as serum or cerebrospinal fluid (CSF), can also detect active infection. PCR on CSF is considered a gold standard testing method, however, on serum samples this method exhibits low sensitivity and may result in false negatives.

Viral culture techniques are considered the gold standard for virus isolation. Classic cell culture techniques require a two to three week incubation period. The rapid cell culture method used by our laboratory detects only active HHV-6 infections and requires an incubation period of 3 days. This method is based on the detection of specific proteins produced only by actively replicating HHV-6. These proteins are detected with a fluorescent stain and specialized microscopic procedures.

Because only active infections of HHV-6 are of concern to clinicians, only the methods that are able to distinguish between active and latent viral infections are of clinical significance and value.

Comparison of Current Viral Detection Methods

Test	Type of Result	Interpretation	Comment
SEROLOGY (Antibodies)
IgG	+ / –	Previous infection	Does not indicate active infection
IgG Titer	Number	Antibody concentration	Not useful in detecting active infections
IgM	+ / –	Recent or current active infection	Demonstrates active infection within past weeks; does not indicate current infection
POLYMERASE CHAIN REACTION
Samples containing cells
Leukocyte	+ / –	Active or latent infection present	Does not prove that an active infection is present
Leukocyte	Number	Active or latent infection present	Does not prove that an active infection is present
Tissues	+ / –	Active or latent infection present	Does not prove that an active infection is present
Samples not containing cells
Serum	+ / –	Active infection present	lower sensitivity
CFS	+ / –	Active infection present	Gold Standard
IMMUNOHISTOCHEMISTRY (anti-structural protein)
Tissue	+ / –	Active infection present	Moderate sensitivity; requires invasive sample
CULTURE (virus isolation)
Blood	+ / –	Active infection present	Gold Standard
Tissue	+ / –	Active infection present	Difficult; low sensitivity
CSF	+ / –	Active infection present	Very low sensitivity
Table design by Wisconsin Viral Research Group, Ltd.