Setting and design
This is a cross-sectional study involving HCWs at the hospital affiliated with the Institute of Medical Science, the University of Tokyo (IMSUT hospital), receiving QFT-GIT for LTBI screening between November 2011 and July 2012. IMSUT Hospital has 135 beds and four negative pressure isolation rooms for patients diagnosed with active tuberculosis accidentally. The hospital is not a tuberculosis referral hospital and has no tuberculosis-specific wards. There were three active tuberculosis patients admitted in the IMSUT hospital in 2007. This is the highest number of annul admission of active tuberculosis cases in the hospital. We reviewed medical records of HCWs and questioned HCWs about exposure to M. tuberculosis and employment length in health care industries. We utilized the questionnaire consisting of the following questions: age, job category, history of contact investigation for tuberculosis, history of living with families who developed active tuberculosis, past history of active/latent tuberculosis infection, length of working in health care industries (general hospitals, tuberculosis referral hospitals, clinics, nursing home), history of working in tuberculosis ward/out patient department (OPD). Eligible subjects had been working as HCWs for more than 6 months and never had received IGRAs for LTBI screening. We excluded the HCWs with history of active tuberculosis. Written informed consent was obtained from subjects before completing the study questionnaire. This study was approved by the Research Ethics Committee, Institutional Review Board of IMSUT (accession number: 24-6, 24-32).
Quantiferon®-TB Gold in Tube
Blood samples for QFT-GIT tests (Cellestis Limited. Australia) were collected at our hospital and QFT-GIT tests were performed at laboratories in the clinical laboratory testing industry (SRL Inc. Japan). Interferon-gamma (IFN-γ) responses to antigens that are at least 0.35 IU/mL greater than the nil control value are considered M. tuberculosis infection (“positive”). We also evaluated a prevalence using the value of 0.10 IU/mL as an alternative cut-off (“intermediate”) as the guideline of committees for the preventions of The Japanese Society of Tuberculosis. If the response is less than a cut-off value and their response to the mitogen positive control is greater than 0.5 IU/mL, the test result is considered negative.
Statistical analysis
Comparisons of proportions were analyzed by the chi-square test or the Fisher exact test and a linear trend in proportions of ordered multiple groups were analyzed by Mantel-extension test. Multiple logistic regression analysis was performed (using JMP 9 software; SAS Institute Inc.) with the following independent variables: male/female, age groups, job category, history of contact investigation for tuberculosis, history of living with families who developed active tuberculosis and history of working in tuberculosis ward or OPD for more than 2 months.