407. Challenges with QuantiFERON-TB assay for large-scale, routine screening of US healthcare workers
Session: Poster Abstract Session: Occupational Health
Thursday, October 3, 2013
Room: The Moscone Center: Poster Hall C
Background: North American occupational health programs that switched from the tuberculin skin test (TST) to Interferon Gamma Release Assays for latent tuberculosis (LTBI) screening are reporting challenges with reproducibility of serial testing results in health care workers (HCWs). However, limited data exist on the interpretation of serial IGRA results in HCWs. Our aims were to evaluate the short-term reproducibility of QuantiFERON-TB Gold In-Tube (QFT) in a large cohort of HCWs and to define a QFT cutoff yielding a conversion rate equivalent to historical TST rates.

Methods: We retrospectively evaluated the QFT results from HCWs with ≥2 QFT tests performed between June 2008 and July 2010 at an academic institution. Outcome measures were proportions of reproducibility, quantitative results, and conversion rates with alternate QFT cutoffs.

Results: 9155 HCWs with ≥2 QFT tests were included in the analysis. Of 7878 individuals with an initial negative result, 4.9% (n=386) converted their QFT result over 2 years. 244 (63.2%) of the HCWs with conversions underwent repeat short-term testing after the first positive result with 63.9% reverting (n=156). An IFN-gamma cutoff of 4.0 IU/mL or higher (current cutoff is 0.35 IU/ml or higher) yielded a conversion rate of 0.4%, equal to the institution’s historical TST conversion rate.

Conclusion: The manufacturer’s and CDC’s definition of QFT conversion results in an inflated conversion rate that is incompatible with our low risk setting.  A significantly higher QFT cutoff value is needed to match the historical TST conversion rate. Non-reproducible conversions in a majority of converters suggested false positive results.
Madeline Slater, MD, Infectious Diseases, Stanford University, Stanford, CA, Madhukar Pai, McGill University, Montreal, QC, Canada, Gary Welland, Stanford University, Stanford, CA, Julie Parsonnet, MD, Stanford University School of Medicine, Stanford, CA and Niaz Banaei, MD, Departments of Pathology and Medicine, Division of Infectious Diseases and Geographic Medicine, Stanford University, Palo Alto, CA

Disclosures:

M. Slater, None

M. Pai, None

G. Welland, None

J. Parsonnet, None

N. Banaei, None

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