The main purpose of preventive therapy is to prevent latent infection from progressing to clinically active TB disease. Therefore, persons with positive tuberculin skin test results who do not have clinically active disease should be evaluated for preventive therapy.

Candidates for Preventive Therapy

Preventive therapy is recommended for the following persons with a positive tuberculin test result regardless of age:

• Persons with known or suspected HIV infection, including persons who inject drugs and whose HIV status is unknown ( <5mm) ^*

• Close contacts of persons with infectious, clinically active TB ( <5mm)^*

• Persons who have chest radiograph findings suggestive of previous TB and who have received inadequate or no treatment ( <5mm)

• Persons who inject drugs and who are known to be HIV negative ( <10mm)

• Persons with certain medical conditions that have been reported to increase the risk of TB (see "Summary of interpretation of tuberculin skin-test results" on page 5) ( <10mm)

• Persons whose tuberculin skin test reaction converted from negative to positive within the past 2 years ( <10mm increase if younger than 35 years of age; <15mm increase if 35 years of age or older) Preventive therapy is recommended for the following persons in high-incidence groups who have a positive tuberculin test result (10 or more millimeters induration), are younger than 35 years of age, and do not have additional risk factors:
  • Foreign-born persons from high-prevalence areas (e.g., Latin America, Asia, and Africa)
  • Medically underserved, low-income populations, including high-risk racial or ethnic groups (e.g., Asians and Pacific Islanders, blacks, Hispanics, and Native Americans)
  • Residents of long-term care facilities (e.g., correctional institutions, nursing homes, and mental institutions)
  • Children younger than 4 years of age

• Other groups identified locally as having an increased prevalence of TB (e.g., migrant farm workers or homeless persons) Persons younger than 35 years of age with no known risk factors for TB should be evaluated for preventive therapy if their reaction to the tuberculin test is <15mm. This group should be given a lower priority for prevention efforts than the groups already listed. Tuberculin-positive staff of facilities in which a person with clinically active disease would pose a risk to large numbers of susceptible persons should also be considered for preventive therapy.

* In some circumstances, persons in these categories may be given preventive therapy in the absence of a positive tuberculin test result. For example, tuberculin-negative children and adolescents who are close contacts of infectious persons and who may be infected but whose skin test result has not yet converted to positive may be given preventive therapy. If therapy is initiated, a repeat tuberculin skin test should be performed 3 months after contact has been broken with the infectious source. If the reaction is positive, therapy should be continued. If the reaction is negative, therapy may be discontinued if contact with the infectious source case continues to be broken. In addition, persons who are immunosuppressed, especially HIV-infected persons may have a negative tuberculin skin test reaction because they are anergic. All HIV-infected persons who are close contacts of persons who have infectious tuberculosis should be administered a full course of preventive therapy -- regardless of tuberculin skin test results or prior courses of chemoprophylaxis -- after the diagnosis of active tuberculosis has been excluded.

Preventive Therapy Regimens

The usual preventive therapy regimen is isoniazid (INH) (for children -- 10 mg/kg daily, for adults -- 5 mg/kg daily up to a maximum of 300 mg daily) for a minimum of 6 continuous months for adults and 6-9 continuous months for children. Twelve months is recommended for persons with HIV infection or other forms of immunosuppression. (Note: Persons with fibrotic infiltrates on a chest radiograph that are thought to represent old, healed TB and those with silicosis who were formerly considered candidates for preventive therapy should receive 4 months of multi-drug chemotherapy.)

To ensure that persons in high-risk groups adhere to therapy, INH can be given twice weekly at a dosage of 15 mg/kg, up to a maximum of 900 mg, using directly observed preventive therapy (DOPT). DOPT refers to the observation by a health care provider of patients as they ingest anti-TB medications. Situations in which patients not receiving DOPT miss appointments or demonstrate other nonadherent behavior should be brought to the attention of the appropriate public health officials. These patients should be considered for DOPT.

Persons given preventive therapy should be monitored monthly for drug side effects, especially signs and symptoms of hepatitis.

The method of DOPT should be based on a thorough assessment of each patient's needs, living and employment conditions, and preferences. The patient and provider should agree on a method that ensures the best possible DOPT routine and that maintains the patient's confidentiality.