520. Poor Reproducibility of Interferon-γ Release Assays For the Diagnosis of Latent TB amongst Healthcare Workers
Session: Poster Session: Hospital-acquired and Transplant Infections
Friday, October 30, 2009: 12:00 AM
Room: Poster Hall A
Background: T-cell based interferon-γ (IFN-γ) release assays (IGRAs) represent an advance in the diagnosis of latent tuberculosis infection (LTBI). However, the reproducibility of positive results and the factors leading to reversions have not been investigated in low-incidence cohorts. To address this void, we investigated the reproducibility of QuantiFERON-TB gold in-tube assay (QFT G-IT) among healthcare workers from our institution.
Methods: All QFT G-IT performed on hospital employees between June 2008 and April 2009 were included in this study. Individuals with no history of LTBI who tested positive (TB Ag minus Nil ≥0.35 IU/ml) underwent repeat testing. Reproducibility of positive results was determined. The association of reversions with spontaneous (Nil) IFN-γ levels or upon stimulation with TB antigen (TB Ag minus Nil) was investigated.
Results: 1,385 (12%) of the 11,929 tests performed were positive. Of the 189 individuals who underwent repeat testing, 121 (65%) were confirmed as positives and 68 (35%) reverted to a negative result. Individuals with low-positive TB-antigen IFN- γ levels were more likely to revert than those with higher levels; of the 90 individuals with IFN-γ levels between 0.35 to 0.70 IU/ml, 51(57%) reverted to negative after repeat testing. High nil levels were also associated with reversion; of 61 individuals with spontaneous IFN-γ level levels between 0.2 and 1 IU/ml, 40 (66%) converted to a negative result when the test was repeated.
Conclusions: QFT G-IT produces a significant number of reversions amongst healthcare workers at low risk for LTBI. High levels of spontaneous IFN-γ and low levels of TB antigen-stimulated IFN-γ production may be useful markers of reversion. Reporting of absolute values instead of qualitative interpretation could be useful to clinicians.
Niaz Banaei, MD1, Victor Herrera, MD2, Kelly Murphy, MD1, Julie Parsonnet, MD, FIDSA, Sharon Perry, PhD1 and  V. Herrera, None..
S. Perry, None..
K. Murphy, None..
J. Parsonnet, None..
N. Banaei, None., (1)Departments of Medicine and Pathology, Division of Infectious Diseases and Geographic Medicine, Stanford University, Palo Alto, CA, (2)Departments of Medicine, Division of Infectious Diseases and Geographic Medicine, Stanford University, Palo Alto, CA