The indications for preventive therapy differ for industrialized countries that have a tuberculosis elimination strategy in mind and low-income countries where the objective is tuberculosis control.

For low-income countries, the Union recommends the identification of children under the age of five years living with any newly identified infectious case.  These children should be examined clinically and if found healthy be given isoniazid.  Some of these children will be infected and thus receive truly preventive chemotherapy while others will have escaped infection.  Tuberculin skin testing is logistically too difficult (and expensive) and is not indicated.

Limiting preventive therapy to this group has several advantages: children tolerate isoniazid well and the risk of selecting drug resistant mutants is small even if there should be clinically not readily apparent tuberculosis as children have generally a very small bacterial load.  If infected, children in this age group also have a high risk of tuberculosis.  Furthermore, if there is a case already in the family, this might be an incentive for improved adherence.  And finally, it is logistically easy to implement as the index case can be asked to administer preventive therapy for the same duration as treatment lasts in the index case.

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