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Published on: May 29, 2016
Performance of a cytomegalovirus IgG enzyme immunoassay kit modified to measure avidity
Harry E Prince1, Mary Lapé-Nixon2, Susan M Novak-Weekley3
1Focus Diagnostics, Cypress, California, USA hprince@focusdx.com.
Insights
The Wampole CMV IgG enzyme immunoassay (EIA) can be modified to measure cytomegalovirus (CMV) IgG avidity, accurately distinguishing recent from past infections. This modification shows high agreement with existing methods and identifies most positive samples as having high avidity.
Area of Science:
- Clinical Virology
- Immunodiagnostics
- Infectious Disease Serology
Background:
- Cytomegalovirus (CMV) IgG avidity testing is crucial for differentiating recent from past infections.
- Existing enzyme immunoassays (EIAs) for CMV IgG avidity require validation for modified platforms.
Purpose of the Study:
- To evaluate the feasibility of modifying the Wampole Laboratories CMV IgG EIA for avidity testing.
- To assess the performance and agreement of the modified Wampole avidity assay compared to a reference method.
Main Methods:
- A panel of 156 serum samples with known avidity status (low, intermediate, high) was used for initial validation.
- Qualitative agreement and correlation of avidity index (AI) values were assessed between the Wampole assay and a laboratory-developed EIA.
- Performance was further evaluated using 470 consecutive clinical samples submitted for CMV IgG avidity testing.
Main Results:
- The modified Wampole assay demonstrated 94% qualitative agreement with the reference avidity assay.
- Avidity index (AI) values showed excellent correlation (r = 0.96, P < 0.0001) between the two assays.
- In clinical samples, 88% of CMV IgG-positive sera exhibited high avidity, with inverse correlation between AI and CMV IgM detection.
Conclusions:
- The Wampole CMV IgG EIA can be effectively modified to measure CMV IgG avidity.
- A significant number of samples submitted for avidity testing may lack prior CMV IgG status confirmation.
- The modified assay reliably identifies high avidity in the majority of CMV IgG-positive clinical samples.
Abstract:
The measurement of cytomegalovirus (CMV) IgG avidity accurately discriminates recent and past CMV infections. We sought to determine if the Wampole Laboratories CMV IgG enzyme immunoassay (EIA) could be modified to measure avidity. The evaluation panel consisted of 156 serum samples we used in 2002 to validate a laboratory-developed EIA, in which 78 serum samples exhibited low avidity, 7 exhibited intermediate avidity, and 71 exhibited high avidity. The qualitative agreement between the two avidity assays was 94% (147/156); all 9 sera with discordant results exhibited intermediate avidity in one of the assays. The avidity index (AI) values in the two assays showed excellent correlation (r = 0.96, P < 0.0001). The definition of high avidity was verified for the Wampole assay by demonstrating high avidity in 91/93 (98%) recently collected CMV IgG-positive/IgM-negative serum samples. The performance of the Wampole avidity assay in a reference laboratory setting was assessed using 470 consecutive serum samples submitted for CMV IgG avidity testing. Surprisingly, 101 serum samples were negative when screened for CMV IgG using the Wampole kit per the package insert; 98 of these 101 serum samples were tested using a CMV IgG chemiluminescent immunoassay, and only 5 were positive. Of the 369 CMV IgG-positive samples, 6% exhibited low IgG avidity, 6% exhibited intermediate avidity, and 88% exhibited high avidity; CMV IgM detection rates were inversely related to AI levels. These findings show that (i) the Wampole CMV IgG EIA can be modified to measure CMV IgG avidity, (ii) many samples are apparently submitted for avidity testing without knowledge of their CMV IgG status, and (iii) most CMV IgG-positive sera submitted for avidity testing exhibit high avidity.
