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Herpes Simplex Virus Type 2 (HSV-2) IgG Inhibition, ELISA

Test code(s) 17171, 17169X, 17170X

This is an outdated version of this FAQ. It was effective 04/20/2012 to 12/28/2014.

The current version is available here.

Question 1. What is the HSV-2 IgG inhibition assay?

A small percentage of samples with low-positive index values in HSV-2 IgG enzyme immunoassays (EIAs) are actually false-positive results.1 The HSV-2 IgG inhibition assay is a method for distinguishing such false-positive results from true-positive results. 

Question 2. How does the HSV-2 IgG inhibition assay work?

The inhibition assay measures the differential abilities of native HSV-1- and HSV-2-infected cell lysates to neutralize patient sample reactivity to recombinant gG2 protein (rgG2), the HSV-2-specific protein used in the HerpeSelect HSV-2 IgG EIA.2 Separate aliquots of the patient sample are incubated with the 2 lysates, then tested in the HerpeSelect HSV-2 type-specific IgG EIA. 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.

Question 3. What are the advantages of the HSV-2 IgG inhibition assay over the Western blot assay?

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 ELISA for detecting IgG seroconversion following newly-acquired HSV-2 infection: the median interval between symptom onset and IgG seroconversion was 21 days for the HerpeSelect ELISA 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.

Question 4. How is the HSV-2 IgG inhibition assay interpreted?

Inhibition results >60% are interpreted as POSITIVE, indicating HSV-2-specific IgG is present. Inhibition results <60% are interpreted as NEGATIVE, indicating HSV-2-specific IgG is not present.

As part of the inhibition assay, sera are re-tested in the routine HerpesSelect 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 ELISA index value is <1.10 for a small percentage of sera. These sera are not considered positive for HSV-2 IgG, and thus the inhibition data can not be interpreted. A comment communicating this information is added to the report in place of a POSITIVE or NEGATIVE result.

Question 5. Is this an FDA-cleared assay?

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 the Focus Diagnostics Reference Laboratory Research and Development group in compliance with CLIA regulations and CAP requirements.

References

  1. Prince HE, CE Ernst, WR Hogrefe. Evaluation of an enzyme immunoassay system for measuring herpes simplex virus (HSV) type 1-specific and HSV type 2-specific IgG antibodies. J Clin Lab Anal. 2000;14:13-16.
  2. Hogrefe W, X Su, J Song, et al. Detection of herpes simplex virus type 2-specific immunoglobulin G antibodies in African sera by using recombinant gG2, Western blotting, and gG2 inhibition. J Clin Microbiol. 2002;40:3635-3640.
  3. Morrow RA, Friedrich D, Krantz E. Performance of the Focus and Kalon enzyme-linked immunosorbent assays for antibodies to herpes simplex virus type 2 glycoprotein G in culture-documented cases of genital herpes. J Clin Microbiol. 2003;41:5212-5214.
This FAQ is provided for informational purposes only and is not intended as medical advice. A physician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.

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