200. Evaluation of the VIDAS TOXO IgG Avidity Assay Compared to a Composite Reference Method (Palo Alto Medical Foundation Toxoplasma Serological Profile) as a Test to Exclude Patients with Acute Toxoplasma gondii Infection of < 4 Months
Session: Poster Abstract Session: Antigen and Antibody Based Diagnostics
Friday, October 21, 2011
Room: Poster Hall B1
Background: The VIDAS® TOXO IgG Avidity (TXGA) qualitative test for the determination of anti-toxoplasma IgG avidity in human serum was compared to a composite reference (modified PAMF Toxoplasma Serological Profile = TSPm) to assess the TXGA assay’s ability to exclude patients with acute T. gondii infection of < 4 months.  Non-immunocompromised pregnant women tested within 4 months of pregnancy and in whom test(s) exclude the possibility of infection within the past 4 months are considered at no risk for congenital toxoplasmosis.

Methods: Samples from 313 subjects were analyzed.  All subjects were required to have a VIDAS® TOXO IgG II assay positive result.  Of the total subjects, 271 were pregnant and 42 were patients who had lymphadenopathy.  All samples were tested with the TXGA assay and the TSPm. The TSPm was used to determine if infection was recent or remote. It  included the Sabin-Feldman Dye Test, the VIDAS® TOXO IgM  assay, and the Differential Agglutination test. ‘Diagnostic truth’ was determined by the TSPm and assigned as infection occurring ≤ 4 months or > 4 months prior to sample collection.

Results: For all subjects (n=313), the frequency of patients for whom acute T. gondii infection of < 4 months could not be ruled out (excluded) was 45.7%, the sensitivity of the TXGA test to serologically diagnose these patients was 96.5%, the specificity was 87.6%, and the “Negative Predicative Value” (“NPV”) was 96.8% compared to the TSPm.  For the pregnant population (n=271), the frequency, sensitivity, specificity and “NPV” were 40.6%, 96.4%, 88.2% and 97.3% respectively. For the lymphadenopathy population (n=42), the frequency, sensitivity, specificity and “NPV” were 78.6%, 97.0%, 77.8% and 87.5% respectively.

Conclusion:  The overall “NPV” was calculated to be 96.8% when using the data from these studies.  As the testing for this study was performed at a reference laboratory and the frequency may not be indicative of typical frequency, this poster is not making any performance claims for NPV.  The frequency of patients infected less than 4 months and the “NPV” obtained at other laboratories may be different. This assay is a valuable diagnostic tool when assessing women’s risk for transmitting T. gondii to the fetus during early stages of pregnancy. 

Subject Category: D. Diagnostic microbiology

Jose G. Montoya, MD, Stanford University School of Medicine, Stanford, CA; Toxoplasma Serology Laboratory, Palo Alto Medical Foundation Research Institute, Palo Alto, CA, Cynthia Press, B.S., Toxoplasma Serology Laboratory, Palo Alto Medical Foundation Research Institute, Palo Alto , CA, Gary Munk, PhD, Clinical Virology, Hackensack University Medical Center, Hackensack, NJ, Stephen Young, Ph D, Microbiology/Virology, TriCore Reference Laboratories, Albuquerque, NM and Bernard Rice, B.S., Clinical Affairs, bioMerieux, Inc., Durham, NC


J. G. Montoya, None

C. Press, None

G. Munk, None

S. Young, None

B. Rice, bioMerieux, Inc.: Employee and Shareholder, Salary

Findings in the abstracts are embargoed until 12:01 a.m. EST Thursday, Oct. 20 with the exception of research findings presented at IDSA press conferences.