Comparison of latent tuberculosis infection rate between contacts with active tuberculosis and non-contacts.

Background: Latent tuberculosis infection (LTBI) rate is usually high in contacts with infectious TB patients. In TB-prevalent country, however, background LTBI rate is already high in general population.

Objective: To compare the LTBI rate between controls and recognized close contacts.

Methods: Between February 2010 and January 2014, 183 controls and 376 contacts with TB infection were enrolled. The tuberculin skin test (TST) and QuantiFERON-TB Gold In-Tube (QFT-GIT) were used to diagnose LTBI.

Results: Higher TST positivity was found in the control group than in the contact group (37.7% vs. 29.9%, P = 0.073). The positive QFT-GIT rate was higher in contacts than in controls (32.6% vs. 24.1%, P = 0.054). A significantly higher positive QFT-GIT rate was found in contacts under 30 years of age than in controls (16.1% vs. 0%, P = 0.005).

Conclusions: In a TB-prevalent country, both TST and QFT-GIT were limited in the diagnosis of recent LTBI in adult contacts probably due to the high background LTBI rate. However, QFT-GIT seems to be better than TST in differentiating LTBI status in contacts younger than 30 year old.