multidrug-resistant TB
Multidrug-resistant TB is tuberculosis caused by bacteria that no longer respond to the two most important first-line drugs, isoniazid and rifampicin. It is as if the strongest, most reliable keys no longer fit the lock — the standard, short, effective treatment simply does not work, and harder options must be used.
Resistance arises chiefly when treatment is incomplete or irregular: surviving bacteria that happen to carry resistance mutations are selected and multiply, while susceptible ones are killed off. A person can also catch already-resistant TB directly from someone else, so MDR-TB is not always the result of a patient's own missed doses.
Treating MDR-TB is longer, more complex and uses second-line drugs that can have more side effects, though newer all-oral regimens have shortened and improved care. Even tougher categories exist, such as extensively drug-resistant TB, where resistance extends to additional key drugs. Rapid molecular tests that flag rifampicin resistance are vital for catching MDR-TB early so effective treatment can begin sooner.
A patient whose TB fails to improve has drug-susceptibility testing showing resistance to both isoniazid and rifampicin; he is referred to a specialist MDR-TB programme.
Resistance to both core drugs defines MDR-TB.
By definition MDR-TB is resistant to at least isoniazid and rifampicin; resistance to rifampicin alone is often used as a rapid surrogate marker prompting MDR-TB management.