multi drugresistant tuberculosis (MDR - TB)

Last reviewed 01/2018

Multidrug resistant tuberculosis

Multidrug resistant tuberculosis (MDR-TB) is defined as Mycobacterium tuberculosis that is resistant to both rifampicin and isoniazid (1).

Additional resistance to any fluoroquinolone (such as ofloxacin or moxifloxacin) and also any one of the three second line injectable agents (amikacin, capreomycin, kanamycin) is known as extensively drug-resistant TB (XDR TB) (1).

There are two principal causes of developing MDR-TB.

  • acquired (secondary) drug resistance
    • caused by inadequate, incomplete or poor treatment quality that allows the selection of mutant resistant strains
  • primary drug resistance
    • when a person is infected with a drug-resistant TB strain
    • transmission of drug-resistant TB occurs exactly in the same way as transmission of drug susceptible TB
    • is seen in up to half of multidrug resistant tuberculosis cases

Previous treatment for tuberculosis has been identified to be the strongest risk factor for MDR TB with 20.5% of previously treated cases presenting with MDR-TB. In addition household contact of a known infected person, younger age, and (putatively) the strain type of tuberculosis are considered as risk factors (1)


  • drug-resistant TB only progresses to active disease in a minority of those infected, and drug-resistant TB infection can remain latent for long periods of time