Future patients might benefit from a combination of linezolid (LIN) and clarithromycin (CLA) in the treatment of Multidrug-resistant and Extensively Drug-resistant Tuberculosis (MDR/XDR-TB) due to possible synergistic activity as shown in in vitro experiments in different Mycobacteria strains. The investigators observed increased LIN serum levels in three cases after combining LIN and CLA of which the investigators described one in a case report (Bolhuis et al). The investigators suggest to conduct a prospective pharmacokinetic study in MDR- and XDR-TB patients to quantify the above described interaction between LIN and CLA.