Guide to the Application of Genotyping to Tuberculosis Prevention and Control

Combining Genotyping and Epidemiologic Data to Improve Our Understanding of Tuberculosis Transmission

If two persons develop TB and are reported to the same TB program at about the same time, the question arises whether they are involved in the same chain of recent transmission. Before genotyping techniques were available, the answer to this question was based on whether epidemiologic links could be identified between the two persons that indicated they were at the same place at the same time during the period that one of them was infectious or whether drug susceptibility results showed the two isolates had the same pattern. Genotyping tests now provide a powerful additional tool to help estimate the likelihood of recent transmission.

This chapter will provide a general overview of genotyping results and how to interpret them in light of other data collected during the initial interview of the TB patient and the subsequent contact investigation. First, we will begin by defining certain key terms. Next, we will consider the simplest outcome, comparing the results of two isolates from two different patients. Then, we will discuss factors to be considered when three or more isolates are found to have matching genotypes. Finally, we will discuss how trends in the number of genotyping clusters have been used as a measure of the frequency of recent transmission.