Analysis of the frequency distribution of tuberculin skin test readings: a tool for the assessment of group contact investigations.

SETTING

The public health tuberculosis control program covering Seattle, Washington, and its surrounding suburban areas.

OBJECTIVE

To describe a tool of potential usefulness in the assessment of transmission of tuberculosis in contact investigations of groups, such as co-workers or schoolmates of an infectious case, with a low prior probability of latent tuberculosis infection (LTBI).

DESIGN

Tuberculin skin test (TST) readings in mm of the group being tested were graphed and compared with the known frequency distributions of TST readings of populations with and without LTBI, the latter including a fraction with non-specific tuberculin reactivity.

RESULTS

Four group contact investigations were analyzed retrospectively with this tool. In two the graphed TST readings of contacts fell within the distribution of a population with LTBI, and suggested that transmission had occurred. In the other two, the graphed readings better fit the distribution of a population with non-specific tuberculin reactivity and suggested that transmission had not occurred.

CONCLUSION

This simple tool to facilitate the determination of whether transmission of tuberculosis has occurred, and who should be offered treatment for LTBI in contact investigations of groups of people, deserves further study.