Screening for Mycobacterium tuberculosis infection among healthcare workers in New Zealand: prospective comparison between the tuberculin skin test and the QuantiFERON-TB Gold In-Tube assay.

A cross-sectional study was used to compare the "QuantiFERON-TB Gold In-tube" assay (QFT-GIT) to the Mantoux tuberculin skin test (TST) as a test for Mycobacterium tuberculosis (TB) infection among healthcare workers in Auckland, New Zealand (NZ). New employees who underwent pre-employment interviews between 1 May 2007 and 18 March 2008 were recruited. Participants completed a detailed questionnaire to assess their risk of TB. All participants were tested by the QFT-GIT and TST. Multiple logistic regression analysis was used to correlate positive results with risk factors for TB and previous BCG. None of the 325 participants were found to have active TB. Approximately 67% had been BCG vaccinated. Positive results for each test were associated with residence in a high incidence country [odds ratio (OR)=6.77; p=0.0005 and 4.48; p<0.0001 for QFT-GIT and TST respectively]. Although positive TST results were associated with "high-risk occupational exposure" to TB [OR=4.13; p=0.016], they were also associated with previous BCG vaccination (OR=5.10; p=0.003). Both tests were associated with at least one well described risk factor for TB infection. The association between positive TST and BCG implies that a high percentage of positive TST results occurred due to non-specific cross-reactivity with BCG. Our findings suggest that among low TB prevalence populations with a high rate of BCG vaccination, the QFT-GIT is more specific than the TST although the superior specificity may be at the expense of some sensitivity.