Failure of isoniazid prophylaxis after exposure to isoniazid-resistant tuberculosis.

We report the failure of isoniazid chemoprophylaxis to prevent the development of active pulmonary tuberculosis and tuberculin conversion in contacts of patients infected with isoniazid-resistant strains of Mycobacterium tuberculosis. Although failure of isoniazid chemoprophylaxis has been reported rarely in contracts of isoniazid-resistant tuberculosis, the cases reported here as well as previous observations by others suggest that isoniazid chemoprophylaxis for contracts of patients with isoniazid-resistant tuberculosis may not be effective and that other agents must be considered. Initial chemotherapy of active pulmonary tuberculosis in contacts of known drug-resistant patients should be based on drug susceptibility studies in the index case.