521. Questionable Effectiveness of Quantiferon TB Gold Test as Screening Tool
Session: Poster Session: Hospital-acquired and Transplant Infections
Friday, October 30, 2009: 12:00 AM
Room: Poster Hall A
Background: CDC recommends use of Quantiferon TB Gold test (QFT-G) in all situations where Tuberculin Skin Test (TST) is used. In December 2007, OSF Medical Center Occupational Health replaced its screening tool TST with QFT-G in-tube assay (QFT-G-IT) for annual screening of Latent Tuberculosis Infection (LTBI) in its health care workers (HCWs). We compared the results of QFT-G-IT with TST.
Methods: This is a prospective study which involved screening of HCWs with QFT-G-IT. The QFT-G-IT is positive if antigen level is >= 0.35 IU/ml. HCWs with a positive QFT-G-IT underwent repeat testing with QFT-G-IT and concurrently with TST within 3 months. Statistical analysis was done using SPSS-16.0.
Results: A total of 6,530 HCWs were screened with QFT-G-IT from January to December 2008. Among them, 287 (4.4%) HCWs had a positive QFT-G-IT. Of 287 QFT-G-IT positive, 123 (42.9%) HCWs previously noted to have TST positive screen. This resulted in 164 (2.5%) HCWs newly diagnosed as LTBI by QFT-G-IT, an extremely high conversion rate (2.5%). Of 164 QFT-G-IT positive 135 were retested by QFT-G-IT and 126 of 135 HCWs were tested by TST. When 135 HCWs were retested with QFT-G-IT, 66 (48.9%) reverted and 69 (51.1%) remained positive. Reversion rate was 48.9% (95CI, 40 to 57%). Of 126 retested with TST, only 2 became positive and 124 were negative. Mean antigen level for those who reverted was 0.70 (95CI, 0.57 to 0.83) and for those remained positive was 1.28 (95CI, 0.89 to 1.67). The difference in the means is statistically significant (p=0.007). The mean antigen level with a positive history of TST is 3.92 (95CI, 3.21 to 4.63), where as with a negative history it is 0.96 (95CI, 0.77 to 1.17). The difference in the means is statistically significant (p=0.000).
Conclusion: Discordance between QFT-G-IT and TST along with high reversion rates with QFT-G-IT raises the concern about the effectiveness of QFT-G-IT as sole screening test. Our results raise the question whether we need a different cut off value for positive QFT-G-IT. In addition to fear and confusion, the over estimated LTBI resulted in significant cost, time and work up of positive case. With available data and discordant results, we cannot rely on QFT-G-IT alone.
Michelle Feigenbaum, RN, BSN1, Sumanth Gandra, MPH, MD1, William Scott, MD, MPH1, Vijaya Somaraju, MD, MPH, Huaping WANG, PhD1, Suzanne Wilton, MS APN, CNP1 and  S. Gandra, None..
V. Somaraju, None..
W. Scott, None..
H. Wang, None..
S. Wilton, None..
M. Feigenbaum, None., (1)University of Illinois College of Medicine, Peoria, Peoria, IL