Determinants of QuantiFERON Plus-diagnosed tuberculosis infection in adult Ugandan TB contacts: A cross-sectional study

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.