Labs have found differences in DST results among the rifamycins, rifampin (RMP) and rifabutin (RBT); the fluoroquinolones, ofloxacin and moxifloxacin; and the second-line injectable agents, kanamycin, amikacin, and capreomycin. In a related finding, isolates resistant to 0.2 mcg/ml INH may be susceptible to higher concentrations. In the Preserving Effective Tuberculosis Treatment Study (PETTS), 32% of RMP-resistant isolates were susceptible to RBT, 41% of kanamycin-resistant isolates were susceptible to capreomycin, and 45% of isolates resistant to 0.2 mcg/ml INH were susceptible to 1.0 or 5.0 mcg/ml (1). Other studies have demonstrated differences in DST results between moxifloxacin and ofloxacin. Whether these in vitro results translate into clinical efficacy is completely unknown. Given the severely limited treatment options in MDR TB, it would be exceedingly useful to know whether these in vitro results translate into evidence for clinically meaningful treatment decisions.