Explain antimicrobial susceptibility

All isolates of M. tuberculosis  should be tested for antimicrobial susceptibility. Initial therapy for most patients with active TB should include at least isoniazid, a rifamycin, pyrazinamide and ethambutol until susceptibility is known. Directly observed therapy (DOT) by a health care worker should be considered for all cases of active TB to minimize failure rates and the risk of emergence of drug resistance. Confirmed multidrug-resistant tuberculosis (MDRTB) should be treated with DOT and a regimen that includes at least 4 drugs to which the organism is susceptible; the total duration of therapy usually is 18 to 24 months.