Susceptibility Testing Methodology

Susceptibility tests can be performed directly, from a

smear positive specimen, or indirectly, from the growth

of colonies from the specimens. The former has the

advantage of measuring the sensitivity prior to cultivation

on laboratory media. The direct method also produces

results more rapidly but; because of uncertainty of the

species of Mycobacterium, and due to less control of the

viable inoculum size, the results require confirmation with

an indirect test, the direct test is not generally utilized.

Three methods make use of critical concentrations to

define drug resistance and can be performed directly or

indirectly:

¾ Absolute concentration method

¾ Resistance ratio method

¾ Proportion method.