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Indiana State Department of Health

Tuberculosis Home > Information for Health Professionals > FAQ's for Health Professionals FAQ's for Health Professionals

Is a History of BCG Vaccination a Contraindication for TB Skin Testing?

A history of BCG vaccination is not a contraindication for placing a TB skin test. Persons born in or who have spent time in countries where TB is prevalent, have received BCG, and have never been screened for TB, should receive a TB skin test as a part of any initial medical evaluation. 

Commonly used in countries with high rates of TB, BCG offers some degree of protection against miliary, bone and joint, and meningeal forms of the disease in infants and young children. However, the protective effect is highly variable and wanes over time. The vaccine can interfere with the interpretation of the TB skin test because it is very difficult to determine whether a positive reaction is due to TB infection or BCG. 

Many foreign-born persons diagnosed with active TB have a history of BCG vaccination. BCG has not been shown to protect against pulmonary disease, and it does not prevent TB infection from occurring. Since a positive skin test reaction due to BCG is unlikely to persist for more than 10 years, a reaction >10 mm in anyone with a history of receiving it should generally be interpreted as TB infection, and a history of BCG vaccination should not influence the decision to treat.

How is infectiousness determined?

Patients should be considered infectious if they: 

  1. are coughing, have sputum smears positive for acid-fast bacilli, or are undergoing any procedure that may induce coughing or aerosol production, 
  2. and are not on anti-TB treatment, have just started therapy, or are having a poor clinical or bacteriologic response to therapy.

Persons with extrapulmonary TB are generally not infectious, unless the site of disease is in the larynx or elsewhere in the airway.

When can a patient be released from isolation?

Patients who most likely have drug-susceptible TB are no longer considered infectious if they meet all of the following criteria:

  • They are on adequate therapy;
  • They have had a significant clinical response to therapy, particularly resolution of cough;
  • They have had 3 consecutive AFB-negative sputum smears from specimens collected 8 - 24 hours apart.

If sputum smears were negative before treatment began, or if they could not be collected because of an inability to cough and produce a suitable specimen, the period of infectiousness may be shortened. 

For patients with multi-drug-resistant disease, the infectious period may be prolonged. Their clinical condition must be closely monitored. For those who are in an institutional or some other congregate setting, TB isolation should be continued until infectiousness is ruled out. 

Since most TB patients are managed as outpatients, it is not necessary to hospitalize them unless their medical condition or living environment warrants it. For example, hospital isolation may be necessary if there are young children or high-risk adults in the home who can’t leave for some reason, or if the patient is a resident of a congregate setting, such as a nursing home, correctional facility, or college dormitory. 

People who live in the patient’s home have most likely already been exposed.  A TB patient who is still AFB smear-positive and who is on an appropriate drug regimen may return home, provided there are no previously unexposed or immunosuppressed persons staying there.