Association of Low Sputum Smear Positivity among Tuberculosis Patients with Interferon-Gamma Release Assay Outcomes of Close Contacts in Japan

Risk prediction and response measures may differ in tuberculosis (TB) patients with low sputum smear positivity for acid-fast bacillus (AFB) compared to those who are smear negative. However, previous studies using the tuberculin skin test (TST) did not show that differences in measures are important. This study compared results of interferon-gamma release assays (IGRA) between contacts of pulmonary TB patients with AFB smear positivity and those with smear negativity using QuantiFERON(®)-TB Gold In-Tube (QFT) assays. Close contacts of TB patients with culture-confirmed infections between April 2010 and December 2012 in Ibaraki, Japan, were enrolled, and 439 Japanese contacts of 129 index TB patients were examined. Adjusted odds ratios of QFT in contacts were 0.68 (95% confidence interval: 0.17–2.8) for AFB scanty patients, 1.12 (0.45–2.8) for AFB 1+, 1.20 (0.48–3.0) for AFB 2+, and 4.96 (1.9–12.9) for AFB 3+, compared to those who were smear negative. Differences in IGRA positivity were not significant between close contacts of TB patients with low positive and negative smears.