Background

The tuberculin skin test is commonly used to diagnose latent tuberculosis infection (LTBI) in resource-limited settings, but its specificity is limited by factors including cross-reactivity with BCG vaccine and environmental mycobacteria. Interferon-gamma release assays (IGRA) overcome this problem by detecting M. tuberculosis complex-specific responses, but studies to determine risk factors for IGRA-positivity in high TB burden settings are lacking.

Methods

We conducted a cross-sectional study to determine factors associated with a positive IGRA by employing the QuantiFERON-TB® Gold-plus (QFT Plus) assay in a cohort of asymptomatic adult TB contacts in Kampala, Uganda. Multivariate logistic regression analysis with forward stepwise logit function was employed to identify independent correlates of QFT Plus-positivity.

Results

Of the 202 participants enrolled, 129/202 (64%) were female, 173/202 (86%) had a BCG scar, and 67/202 (33%) were HIV-infected. Overall, 105/192 (54%, 95% CI 0.48–0.62) participants had a positive QFT Plus result. Increased risk of QFT-Plus positivity was independently associated with manual employment/unemployment vs. professional employment (adjusted odds ratio (aOR) 2.18, 95% CI 1.01–4.72), a family vs. non-family relation to the index patient (aOR 2.87, 95% CI 1.33–6.18), living in the same vs. a different house as the index (aOR 3.05, 95% CI 1.28–7.29), a higher body mass index (BMI) (aOR per additional kg/m2 1.09, 95% CI 1.00–1.18) and tobacco smoking vs. not (aOR 2.94, 95% CI 1.00–8.60). HIV infection was not associated with QFT-Plus positivity (aOR 0.91, 95% CI 0.42–1.96).

Conclusion

The prevalence of IGRA positivity in this study population was lower than previously estimated. Tobacco smoking and raised BMI are hitherto unappreciated determinants of LTBI risk in this setting.

A cross-sectional study to determine factors associated with a positive IGRA by employing the QuantiFERON-TB® Gold-plus (QFT Plus) assay in a cohort of asymptomatic adult TB contacts in Kampala, Uganda. Multivariate logistic regression analysis with forward stepwise logit function was employed to identify independent correlates of QFT Plus-positivity.

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