Guide to the Application of Genotyping to Tuberculosis Prevention
and Control
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Developing a Tuberculosis Genotyping Program
Establishing Procedures for Submitting Additional Isolates from
the Same Patient
Under normal circumstances, the genotyping laboratories will genotype
only one isolate from each patient with TB. Like drug-susceptibility
test results, there is usually no benefit in testing additional
isolates. There are exceptions to this rule, however, and some of
them are discussed in the following sections.
Evaluate Likelihood of Exogenous Reinfection versus Relapse
Patients who recently completed therapy but subsequently become
ill may have been infected with a different strain of M. tuberculosis
(i.e., exogenous reinfection), or they may have had a relapse of
their initial infection that was caused by the same M. tuberculosis
strain. Genotyping the subsequent isolates will help distinguish
between these two possibilities.
Assess Discrepant Drug Susceptibility Test Results
A second patient isolate should be submitted for genotyping when
drug susceptibility results differ substantially among isolates
from the same patient. Although discrepant results can indicate
that the patient’s isolate has developed secondary drug resistance,
the results may be caused by a false-positive culture. Genotyping
the subsequent isolate will help distinguish between these two possibilities.
Evaluate Questionable Genotyping Results
A second isolate from the same patient may be considered for submission
when the genotyping results from the first isolate are not compatible
with epidemiologic information. For example, if two TB patients
were identified as having spent a prolonged period of time together
in the same place but their isolate genotype patterns do not match,
a second isolate should be submitted for genotyping to evaluate
the possibility that an error occurred. Such errors can occur during
the following steps: submission of the isolate, processing of the
isolate at the genotyping laboratories, or transmission of the results
to the TB program. If the genotyping results for the second isolate
are identical to those of the first isolate, the most likely explanation
is that the initial epidemiologic information was incorrect or not
relevant to the actual chain of transmission.
Last Reviewed: 05/18/2008 Content Source: Division of Tuberculosis Elimination
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
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