**3. The problem of LTFU**

The patients who were LTFU have not completed the treatment regime. This can cause serious public health problems because these patients are at higher risk of drug resistance [11]. They continue to spread the potentially resistant bacilli to the public, infecting the public. This has been proved in a Bayesian mapping where LTFU has served as an important indicator for the distribution of TB patients [12]. Therefore, LTFU should be one of our primary concerns in the battle against TB.

Even just a single case of LTFU could cause an outbreak of TB, as observed in countries with low incidence such as Norway [13, 14], USA [15], and Austria [16]. In such outbreaks, the index cases are mostly immigrants, spreading the infection to their families, friends, and other social networks. To further visualize this problem, we need to look into the proportion of LTFU among different countries in the world.
