The proportion method was originally proposed by Canetti and colleagues, and modified later; it is the most common method used for testing the susceptibility of M. tuberculosis complex isolates. In this method, the inoculum used is monitored by testing two dilutions of a culture suspension, and the growth (that is, the number of colonies) on a control medium without an anti-TB agent is compared with the growth (the number of colonies) present on a medium containing the critical concentration of the anti-TB agent being tested; the ratio of the number of colonies on the medium containing the anti-TB agent to the number of colonies on the medium without the anti-TB xiii agent is calculated, and the proportion is expressed as a percentage. A 1% critical proportion is used to differentiate the proportion of resistant organisms within a particular sample that is used to determine resistance to a particular drug.

OTHER DEFINITIONS