Multidrug-resistant (MDR) isolates of Mycobacterium tuberculosis complex (MTBC) are defined by resistance to at least rifampin (RMP) and isoniazid (INH). Rapid and accurate detection of multidrug resistance is essential for effective treatment and interruption of disease transmission of tuberculosis (TB).T wo essential drugs for the first-line treatment of tuberculosis (TB) are rifampin (RMP) and isoniazid (INH). Isolates of Mycobacterium tuberculosis complex (MTBC) that are resistant to at least both these drugs are classified as multidrug resistant (MDR). Rapid and accurate detection of resistance to either RMP or INH is crucial for selection of treatment regimens and public health interventions. In 2012, the Centers for Disease Control and Prevention (CDC) reported 9,945 cases of TB in the United States (http://www.cdc.gov/tb/statistics/reports/2012/default.htm). For 7,188 of these cases, for which initial drug susceptibility to firstline antituberculosis drugs was reported, 660 (9.2%) were at least INH resistant, and 83 (1.2%) were MDR. The American Thoracic Society, CDC, and the Infectious Diseases Society of America have issued guidelines for treating INH-monoresistant TB (1). Early detection of RMP resistance, which correlates well with multidrug resistance, is critical for the initiation of effective second-line treatment regimens and interruption of disease transmission.CDC offers the molecular detection of drug resistance (MDDR) for mutations associated with resistance to RMP at the RMP resistance-determining region (RRDR) of the rpoB locus and with resistance to INH at the katG and inhA loci (2, 3). Other loci that are examined are embB (EMB resistance), pncA (pyrazinamide resistance), gyrA (fluoroquinolone resistance), rrs (kanamycin, amikacin, and capreomycin resistance), tlyA (capreomycin resistance), and eis (promoter region mutations associated with kanamycin resistance) (2). MDDR is available by request in coordination with state public health laboratories (PHL) for M. tuberculosis isolates and sediments meeting submission criteria (http://www.cdc.gov/tb/topic /laboratory/MDDRsubmissionform.pdf). Although molecular testing can rapidly detect mutations associated with drug resistance, it should complement, not supersede, conventional phenotypic drug susceptibility testing (DST) (4). Therefore, all submissions to MDDR undergo growth-based DST for a full panel of first-line and second-line drugs (4). Submitters receive a preliminary report with molecular test results and a final report upon completion of DST, with both the molecular and DST results and interpretive comments.In this study, we examined the concordances between molecular testing and DST for RMP and INH to determine the performance characteristics of CDC's MDDR service for rapid detection and confirmation of MDR TB. Through an electronic survey using a secure data collection instrument, we collected phenotypic DST results from state and local PHL for isolates submitted for testing at CDC. We examined the concordances between molecular ...

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