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Moxifloxacin Treatment of Tuberculosis

We are pleased to see that the study of Pletz et al. (6) confirms our earlier observations that moxifloxacin is active in patients with pulmonary tuberculosis with positive sputum smears (3). However, some of their results differ from ours, and we believe that this is due to the method that they adopted. In our study, we recruited groups of 15 subjects (3) and found that moxifloxacin was significantly less active than isoniazid when the time to reduce the viable bacillus count by 50% (vt50) was calculated (1), but like Pletz et al., we found no significant difference between the two drugs when early bactericidal activity (EBA) was calculated (3, 5, 6). The most important reason for this difference is the number of patients in the groups recruited by Pletz et al. is small, and thus, the absence of a statistically significant difference cannot be construed as indicating biological equivalence. Second, the EBA methodology that Pletz et al. use is a variation of the standard method (4), which calculates the value for EBA over 5 days rather than 2 days and has limited power to distinguish between different drugs (5). The EBA methodology produces large confidence intervals, and for this reason, studies have shown that the results for isoniazid are poorly reproducible between centers (7). Third, by grouping the data from different patients together, the errors are enlarged. Viable counts of mycobacteria from sputum vary enormously between patients (often by more than 1 order of magnitude) and from day to day. As the EBA measure is half (or a fifth) of the ratio between the day 0 and day 2 (or day 5) values expressed logarithmically, small variations in sputum mycobacterial viable counts result in large variations in the EBA values as found in the study of Pletz et al.