Extensively drug resistant TB (XDR TB) is a rare type of MDR TB. XDR TB is defined as TB that is resistant to isoniazid and rifampin, plus resistant to any fluoroquinolone and at least one of three injectable second-line drugs.

Because XDR TB is resistant to first-line and second-line drugs, patients are left with treatment options that are much less effective. XDR TB is of special concern for people with HIV infection or other conditions that weaken the immune system. These people are more likely to develop TB disease once they are infected and have a higher risk of death once they develop TB.