Having just two categories for all those people who have MDR or XDR TB, is possibly under representing the extent of resistance to the second line TB drugs, as well as causing confusion as people in an ad hoc way adopt new terms.

A study reported in 2012, showed that of 1,278 patients with MDR TB, some 43.7% had resistance to at least one second line drug. They did not though have the specific resistance to two of the classes of second line drug, necessary for them to be categorised as XDR TB, which in this study was at a level of 6.7%

It could be useful to have a new category of "MDR plus TB" for people with MDR TB, but who also have some resistance to second line drugs, but not sufficient for them to be categorised as having XDR TB. If monitored by WHO and others, it could enable more information to be available about the extent to which resistance is developing to the second line drugs.

The adoption by WHO of a new category of MDR plus TB, would also reduce the confusion caused by the current use by some people of the term "pre XDR-TB".