Herpes Simplex Virus Type 2 (HSV-2) IgG Inhibition Assay

Some low-positive index value results (1.10-3.00) on HSV-2 IgG chemiluminescent immunoassays (CIAs) may be false-positive results.¹ The HSV-2 IgG inhibition assay is a method that may be used to help distinguish such false-positive results from true-positive results.

The HSV-2 IgG inhibition assay measures the differential abilities of lysates of cells infected with HSV-1 or HSV-2 to neutralize patient sample reactivity to recombinant gG2 protein (rgG2), the HSV-2–specific protein used in the HerpeSelect HSV-2 IgG ELISA.² Separate aliquots of the patient sample are incubated with the 2 lysates, then tested in the HerpeSelect HSV-2 type-specific IgG ELISA. The index value of serum pre-incubated with HSV-2 lysate is then compared to the index value of serum pre-incubated with HSV-1 lysate. The HSV-1 lysate controls for any non-gG2 reactivity present in the serum sample; thus, any inhibition that is detected reflects only the absorption of gG2 reactive antibodies by the native gG2 present in the HSV-2 lysate. An example of how the inhibition value is calculated is shown below:

Index value for serum pre-incubated with HSV-2 lysate = 0.23

Index value for serum pre-incubated with HSV-1 lysate = 2.45

Inhibition = [1 - (0.23/2.45)] x 100 = [1- 0.09] x 100 = 0.91 x 100 = 91%

Inhibition values >60% indicate true HSV-2-specific IgG reactivity

The HSV-2 IgG Western blot assay has historically been considered the gold standard for identifying HSV-2 IgG reactivity. However, Ashley-Morrow et al have clearly demonstrated that the HSV-2 IgG 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.³ There is thus a window of approximately 47 days in which the Western blot assay may give false-negative confirmatory results. In contrast, true-positive specimens in this window are positive (>60% inhibition) in the HSV-2 IgG inhibition assay. In addition to increased sensitivity, the turnaround time for the inhibition assay is shorter than that of the Western blot assay.

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.

As part of the inhibition assay, sera are re-tested in the routine HerpeSelect HSV-2 IgG ELISA (ie, an aliquot of the sample is diluted in specimen diluent rather than HSV-1 lysate or HSV-2 lysate); the resulting routine index value is <1.10 for a small percentage of sera. Because these sera are not considered positive for HSV-2 IgG, the inhibition data cannot be interpreted. A comment communicating this information is added to the report in place of a positive or negative result.

Although an FDA-cleared kit is used as the basis for the assay, the procedure employs modifications that were validated in extensive studies performed by Quest Diagnostics in compliance with Clinical Laboratory Improvement Amendments of 1988 (CLIA) regulations and College of American Pathologists (CAP) requirements.