Contact investigation has three main objectives. In order of priority these are:
1. Identify and initiate treatment for secondary cases of active TB disease
Typically, 1-2% of close contacts are found to have active disease at the time of contact investigation; the proportion is about 3.3% for smear-positive pulmonary patients.7,10 Early identification and treatment reduces the morbidity and mortality risk of TB disease for these individuals and rapidly reduces the risk of further transmission to others. This objective is particularly critical for contacts who are vulnerable to rapid progression if infected, such as children less than 5 years of age or those with significant immune suppression.9,11
2. Identify and treat the infectious source patient if the index patient is less than 5 years old.
TB disease in a young child (whether pulmonary or extra-pulmonary) is a sentinel event. The younger the child, the more likely this reflects recent transmission, usually from an undiagnosed adolescent or adult in the household, or other caregiver close to the child (see Chapter 9: Pediatric Tuberculosis).9,12,13 Source-case investigation (also known as “reverse contact investigation”) should be carried out when TB disease is diagnosed in any child less than 5 years old.14,15 Source investigation should also be done when a cluster of tuberculin skin tests (TST) conversions is identified in an institutional setting with no known source patient, and may be considered for patients with pleural TB if TB program resources allow.16 However, source investigations usually give very low yield; even for patients less than 5 years old a source case is identified in less than half of investigations.12 Source-case investigation is not advised for individuals of any age with LTBI identified on a routine screening.
3. Identify contacts with LTBI in order to offer preventive treatment.
Without treatment, about 5% of newly infected contacts will develop active disease within two years of exposure. Thus, a well-functioning contact investigation and follow-up program can reduce morbidity and mortality among infected contacts, and over time can contribute substantially to TB elimination.
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