Effective antimicrobial treatment of Mycobacterium avium-intracellulare complex (MAC) has not been established. Clarithromycin (CAM) is an extremely important drug in treatment regimens of MAC diseases. Except for monotherapy, the clinical features of CAM resistance are not clear. We investigated the clinical background of CAM resistance of pulmonary MAC disease patients. Minimum inhibitory concentrations (MICs) of CAM to 283 strains of M. avium and 58 strains of M. intracellulare were determined by drug susceptibility test using BrothMIC NTM. All 243 M. avium isolates from untreated patients except one isolate were susceptible to CAM. We also examined CAM susceptibility of 40 pulmonary disease patients who received chemotherapy including CAM during a period of over 6 months. Seventeen patients (43%) were resistant to CAM. All (17/17) resistant patients were treated with CAM monotherapy. However 8 of the 23 (35%) susceptible patients were also treated with monotherapy. Many resistant patients were treated with high dose CAM monotherapy and were classified as the non-nodular bronchiectasis type. However 7 of 8 susceptible patients despite long-term monotherapy were the nodular bronchiectasis type. High dose CAM monotherapy and non-nodular bronchiectasis subtype were considered to be risk factors for CAM resistance.
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