Herpes Simplex Virus (HSV) Type-Specific IgG Antibodies, With Reflex to HSV-2 Inhibition

Not necessarily. A negative result does not rule out infection, because the specimen may have been collected before antibodies reached detectable levels.^1,3 This assay can detect HSV IgG antibodies as early as 3 weeks after the onset of symptoms.³ However, people seroconvert at different rates, and some do not seroconvert for 6 months or longer.³ If recent acquisition is suspected, repeat testing is recommended 12 weeks after the presumed exposure.¹

Additionally, a very small percentage (0.2%) of HSV-2 isolates lack glycoprotein G (gG).⁴ When the infecting HSV isolate is gG deficient, a false-negative result may occur. 

A positive result suggests a recent or past HSV infection. Infection with HSV causes latent lifelong infection. Reactivation and viral shedding can be associated with symptoms that may be subclinical or mild enough to be unrecognized. 

A small percentage of positive HSV-2 IgG results may be false-positive.^1,5 Further testing may be appropriate for patients with low HSV-2 IgG index values or unexpected positive HSV-2 IgG results. At Quest, the HSV-2 IgG inhibition assay is an ELISA-based immunoassay performed to distinguish a false-positive from a true-positive result. 

Some low-positive index values obtained with the HSV-2 IgG assay may be false-positives.⁵ The HSV-2 IgG inhibition assay is an ELISA-based immunoassay used to confirm HSV-2 IgG screen positive results.  

The HSV-2 IgG inhibition assay measures the ability of HSV type lysate to specifically neutralize reactivity to recombinant gG2 protein (rgG2), the antigen used to detect HSV-2 IgG antibodies.⁶

Patient sera are pre-incubated with HSV-1 and HSV-2 lysate separately, then tested using the HerpeSelect HSV-2 type-specific IgG ELISA. A baseline HSV-2 IgG is measured with sera that has not been pre-incubated with lysate. Baseline IgG is compared to IgG levels in the presence of HSV-1 and HSV-2 lysate to calculate percent inhibition.

Inhibition results >60% are interpreted as positive, indicating that HSV-2-specific IgG is present. Inhibition results <60% are interpreted as negative, indicating that HSV-2-specific IgG is not present.⁶

Occasionally, samples that tested positive on the HSV-2 IgG screen assay using the Diasorin Liaison XL will not have a measurable baseline IgG level on the inhibition assay using the HerpeSelect HSV-2 type-specific IgG ELISA. Therefore, a percent inhibition cannot be calculated for those samples. This is more likely to happen with samples that have a low screen-index value.

Alternative confirmatory testing by Herpes Western blot is available directly through the University of Washington.

If confirmatory testing cannot be performed, patients should be counseled about the limitations of available testing before obtaining serologic tests, and healthcare providers should be aware that false-positive results occur.¹

The HSV-2 IgG inhibition assay is more sensitive and has a shorter turnaround time than the Herpes Simplex Western Blot performed at the University of Washington. The Herpes Simplex Western Blot assay has historically been considered the gold standard for identifying HSV-2 IgG reactivity.

Ashley-Morrow et al have demonstrated that the Herpes Western blot is less sensitive than the HerpeSelect HSV-2 IgG assay for detecting IgG seroconversion following newly acquired HSV-2 infection: the median interval between symptom onset and IgG seroconversion is 21 days for the HerpeSelect assay versus 68 days for the Western blot assay.⁷ There is thus a window of approximately 47 days in which the Western blot assay may give false-negative results. In contrast, true-positive specimens in this window are positive in the HSV-2 IgG inhibition assay.

Both the Herpes Simplex Western Blot and the HSV-2 IgG inhibition assays are CLIA-validated laboratory-developed tests (LDTs) and are not available as commercial kits. The HSV-2 IgG inhibition assay is only performed at Quest Diagnostics and is available as reflex testing from an HSV-2 IgG screen that has an index value that meets the criteria for confirmatory testing (index value 1.1-6.0).