1037. "Wobblers" - Variability in QuantiFeron Gold In-Tube Test Results: Not as Variable as they Seem
Session: Poster Abstract Session: Diagnostic Procedures in Clinical Practice
Saturday, October 22, 2011
Room: Poster Hall B1
  • IDSA_Poster_Wobblers 1011 (2).pdf (558.4 kB)
  • Background: The Palo Alto Veterans Hospital ran QuantiFeron Gold In-tube tests on 810 health care workers in two consecutive years for the detection of latent tuberculosis infection. Because our workforce demonstrates a high background incidence of latent tuberculosis infection, such testing is performed every year irrespective of initial test result. Some patient/employees were noted to have discordant, or "wobbling" test results between the different testing years.

    Methods: A retrospective results review was performed on serial test results and on all results which "wobbled" between positive and negative during 2009 and 2010. The TBag-nil level was recorded for these patient/employees and plotted to determine possible predictive value of the TBag-nil levels.

    Results: 58 (7.2%) of 810 serially tested employees had discordant results between years, either initially "positive" and subsequently "negative" or the reverse. Forty-eight (83%) of the "wobblers" had TBag-nil levels of <1.0 (on a scale of 1 to approximately 15). Twenty-two (38%) had TBag-nil levels <0.51; 26 (45%) had TBag-nil levels between 0.51 and 1.0; and 10 (17%) had levels >1.01.

    Conclusion: The IGRAs are giving clnicians an opportunity to determine latent tuberculosis infection with at least 92% confidence. There is a population of individuals whose TBag-nil levels hover near the cutoff point of 0.35, with 83% of our "wobblers" having a level <1.0. It is these individuals whose result may drop above or below this cutoff in serial testing, causing apparently discordant results. Such transient, minor, fluctuations in the TBag-nil levels could reflect test/reagent variability or biologic variability or both. As per CDC guidelines, the QuantiFeron Gold In-Tube quantitative results should be released to the provider so that an informed decision can be made regarding retesting versus treatment options.

    Subject Category: J. Clinical practice issues

    Wendy Thanassi, MA, MD1,2, Lorraine Guerrera, MLS, SI3, Virginia Pirrotta, MS, MLS3, Grace Cheung, MLS3 and Irene Iuppa, BA4, (1)Medicine, Palo Alto Veterans Administration Hospital, Palo Alto , CA, (2)Emergency Medicine, Stanford Hospital and Medical Center, Palo Alto, CA, (3)Immunology, Palo Alto Veterans Administration Hospital, Palo Alto, CA, (4)Palo Alto Veterans Hospital, Palo Alto, CA


    W. Thanassi, None

    L. Guerrera, None

    V. Pirrotta, None

    G. Cheung, None

    I. Iuppa, None

    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.