Differences in the prevalence of infection are apparent not only within but also between countries, as these simultaneously conducted surveys from Africa show.

In order to increase the specificity of the tuberculin skin test result (at the cost of loss in sensitivity), age-specific prevalence of >15 mm induration was compared here for 4 African countries.  The figure under-estimates thus the prevalence of infection, but is more informative (because of the higher specificity) about the relative differences between countries.

Quite clearly, the age-specific prevalence of tuberculous infection varies widely between these 4 countries.

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