Duration of exposure to a case of smear positive tuberculosis and rates of positive whole-blood interferon gamma test and tuberculin skin testing

Abstract number: 1732_313

Lillie P., Thaker H., Anderson G., Cartlich K., Goodrick M., Meigh R., Barlow G., Newton A., Moss P.

Objective: Duration of exposure to tuberculosis (TB) is a major risk factor for transmission of the disease. Current WHO guidelines, based on transmission of TB on aeroplane flights, recommend an 8 hour period as a cut off for screening. However this was before the routine use of IFN-g assays. We compared Mantoux skin testing with QUANTIFERON-TB GOLD (QFN) in a hospital associated outbreak, stratifying patients by duration of contact.

Methods: Hospital contacts of the index case were screened in a sequential manner (close contacts sharing the same room for ≥8 hours, contacts on the same ward for ≥8 hours and contacts on ward <8 hours). Contacts were screened by TB specialist nurses with both Mantoux skin testing and QFN. A 15 mm cut off for reading Mantoux tests was used. Data on symptoms, duration of exposure and prior BCG vaccination were recorded.

Results: 36 contacts were referred for screening, of whom 29 were screened, 28 with both QFN and Mantoux. 20 patients had ≥8 hours exposure, with 8 (40%) having a positive QFN test compared with 3 (15%) a positive Mantoux test. Of the 9 patients with <8 hours exposure 1 (11.1%) had a positive QFN test, 2 (22.2%) having positive Mantoux tests. 60% of positive Mantoux tests were known to have had BCG vaccination, with 22.2% of QFN positive patients having had BCG.

QFN and TST results stratified by 8 hour exposure

 Duration of exposure
<8 hours>8 hours 
QFN +ve18
QFN -ve812
TST +ve23
TST -ve717

Conclusion: QUANTIFERON-TB GOLD testing in a hospital associated outbreak of TB correlates well with ≥8 hour duration of exposure, in agreement with WHO guidelines. Bigger prospective studies, comparing IFN-g assays with Tuberculin skin testing, are needed to confirm this finding.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Location: ICC, Munich, Germany
Presentation type:
Back to top