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.  

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