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
Handouts
  • 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

    Disclosures:

    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.