Environmental mycobacteria live in soil and water. As this example of M. avium complex shows, this species is more abundant in soil with an acid pH than in soil with a neutral pH. Different environmental mycobacteria that might sensitize humans may prefer different environmental conditions.
Because of this variation, the frequency of cross-reactions is not predictable for a given geographic location, making the specificity of the tuberculin skin test variable, while the sensitivity is reasonably well characterized. The utilization of the same cut-off point in every situation to denote infection is thus not justified.
