Pulmonary tuberculosis misdiagnosed as extensively drug-resistant tuberculosis due to selection of a nontuberculous mycobacterial colony from a preculture dish used for a drug-susceptibility test.

Drug-susceptibility test (DST) is important for tuberculosis care; however, there are several pitfalls with the procedure. A 70-year-old woman was diagnosed with extensively drug-resistant tuberculosis based on the result of a DST using microdilution method. Because she had no history of medication for tuberculosis and the sputum acid-fast bacillus smear test turned negative during standard treatment, identification of the strain used for DST was performed. Consequently, the strain was found to be M. intracellulare. It was assumed that a colony of M. intracellulare that had existed in the preculture solid medium was selected and used for the DST.