In the United States, it was observed that the distribution of tuberculin skin test reaction sizes varied by geographic location.

In Minnesota (a northern State), very few persons had small reactions around 5 mm.  In North Carolina (a southern State), a much larger proportion had such reaction sizes.

Denoting a reaction of 5 mm or more as indicative for tuberculous infection would include few persons in Minnesota, but many in North Carolina who have actually no tuberculous infection.  Apparently, agents inducing non-specific cross-reactions are more frequently found in some than in other areas.

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