Extensively Drug-Resistant Tuberculosis Is Worse than Multidrug-Resistant Tuberculosis: Different Methodology and Settings, Same Results

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Abstract
To the Editor—We read with interest the article by Kim et al. [ 1] about the impact of extensively drug-resistant (XDR) tuberculosis (TB) on treatment outcomes of non-HIV-infected patients affected by multidrug-resistant (MDR) TB. Kim et al. [ 1] found, with univariate analysis, that patients with XDR TB had a borderline-significant higher probability of treatment failure and death than did patients with MDR TB ( table 1). Multivariate analysis confirmed that XDR TB is a poor independent prognostic factor for treatment failure (OR, 4.46; 95% CI, 1.35–14.74). Two studies from our group had previously reached similar conclusions [ 2, 3]. Our first study found that patients with XDR TB in Italy and Germany, compared with patients with MDR TB, had a 5-fold increase in the risk of death (relative risk, 5.45; 95% CI, 1.95–15.27; P<.01), required longer hospitalization (mean duration ± SD, 241.2±177.0 vs. 99.1±85.9 days; P<.001), had longer treatment duration (30.3±29.4 vs. 15.0±23.8 months; P<.05), and, for the few patients whose sputum and smear converted from positive to negative, a longer time to smear or culture conversion (P<.01) [ 2]. The second study (including additional patients from Estonia and Russia) found that patients with XDR TB had a relative risk of 1.58 to die or have treatment failure, compared with patients with MDR TB resistant to all first-line drugs (95% CI, 1.14–2.20; P<.05), and a relative risk of 2.61 (95% CI, 1.45–4.69; P<.001), compared with patients with MDR TB for whom susceptibility to ⩾1 first-line drug still existed [ 3]. Interestingly, the results of the studies from the 2 groups are consistent, although the definitions used were slightly different: Migliori et al. [ 2] used the World Health Organization definitions of treatment success and failure [ 4, 5], and Kim et al. [ 1] applied the definitions proposed by Laserson et al. [ 6]. Furthermore, Kim et al. [ 1] (and not Migliori et al. [ 2]) included death with treatment failure. To make a contribution toward the use of standardized definitions and to allow a better comparison of the data from the 2 groups, we recalculated our treatment outcomes from the 4-country study on the basis of the methodology of Kim et al. [ 1] ( table 1). With the univariate analysis, patients with XDR TB had a significantly higher probability of treatment failure than did patients with MDR TB (relative risk, 2.19; 95% CI, 1.31–3.66; P=.002). According to our data, patients with XDR TB had a higher probability of death and treatment failure than did patients with MDR TB, even when the 2 outcomes were analyzed separately ( table 1). With the multiple regression analysis, the presence of XDR was an independent risk factor for both death (OR, 2.07; 95% CI 1.05–4.05; P<.034) and treatment failure (OR, 2.37; 95% CI, 1.14–4.89; P<.02).