Guide to the Application of Genotyping to Tuberculosis Prevention
Applying Genotyping Results to Tuberculosis
Suspected False-Positive Culture Investigations
The goal of a suspected false-positive culture investigation is
to confirm (or refute) the suspicion that one or more of the patients
in a genotyping cluster has been falsely diagnosed with TB on the
basis of a false-positive culture result.
The first step of an investigation of suspected false-positive
cultures is to gather information to verify or refute that suspicion.
Clues that are helpful in deciding if a false-positive culture occurred
fall into two categories. The first type of clue comes from an analysis
of the path that the suspect specimens took from collection through
the final laboratory processing step to identify possible common
collection or processing points and common times that could have
resulted in cross-contamination. The second type of clue comes from
reviewing medical records to identify patients in the cluster who,
despite a diagnosis of TB, do not fit the typical clinical picture
of the disease. The TB program should alert providers of any patient
with a suspected false-positive culture result in order to determine
the patient’s clinical status and if the patient is receiving anti-tuberculosis
Possible sources and locations of cross-contamination of clinical
specimens include bronchoscopes, sputum collection areas, and laboratory
processing steps. The laboratory that reported the suspect culture
should be contacted and asked to provide information on all M.
tuberculosis isolates from any specimens collected or processed
on the same day or within a few days of the suspect isolate. If
a contaminated bronchoscope or other instrument or a sputum collection
booth is implicated in cross contamination, the respective health-care
facility should be contacted and asked to provide information on
other patients who were examined with the same instrument or had
sputum collected in the same location. Information on all implicated
specimens should be recorded and compared to identify potential
overlap that could have resulted in cross-contamination. Realize
that M. tuberculosis can remain viable in certain environments
Clues to patients who may have been misdiagnosed with TB include
patients diagnosed with pulmonary TB but who have normal chest radiographs,
patients who were diagnosed with a different condition before the
suspect M. tuberculosis culture results were reported, patients
who have not been started on treatment for TB or who were started
only after the culture results were reported, and patients who have
had multiple specimens evaluated for M. tuberculosis but
only one positive specimen. Finally, if genotyping results of isolates
from suspected false-positive cultures were not the basis of initiating
the investigation, those results should be obtained as soon as possible.
All M. tuberculosis isolates that were collected or processed
at the location or during the time that the cross contamination
might have occurred should be genotyped.
Deciding whether to request RFLP analysis on isolates identified
as part of a false-positive culture investigation depends on the
strength of the available evidence. Experience has shown that if
the laboratory or the health-care provider suspected a false-positive
culture before the PCR genotyping results indicated a match, the
PCR results are sufficient to confirm the presence of a false-positive
culture. If, on the other hand, the PCR cluster results were the
first indication of a problem, RFLP analysis of the clustered isolates
should be requested.
A suspected false-positive culture result should be considered
“confirmed” as being false if a) all three genotyping methods show
a match with the presumed source of the false-positive culture (or,
if a previous suspicion of a false-positive culture existed and
the PCR genotyping methods show a match), b) the investigation confirmed
that the suspect isolates were processed at the same time or collected
in the same location or with the same instrument, c) there is no
other likely explanation for the findings, and d) the presumed misdiagnosed
patient does not have a clinical picture consistent with TB. If
critical specimens are unavailable for genotyping but all the other
criteria are met, a suspected false-positive culture result should
be considered “likely.”
A suspected false-positive culture result should be considered
“unlikely” to be false (i.e., it is likely that the culture results
of M. tuberculosis are correct) if the genotyping results
do not show a match between the isolate from the suspected false-positive
culture and other isolates processed at the same time or collected
at the same place.
If the investigation leads to the conclusion that a false-positive
culture result is confirmed or likely, the next steps are a) to
identify which patients actually have TB and which patient or patients
were misdiagnosed on the basis of false-positive culture results,
b) to alert the involved health-care providers so that they can
correctly diagnose and treat the misdiagnosed patient, and c) to
alert the involved clinical laboratory or health-care facility so
that the cause of the false-positive culture can be determined and
Identifying Sources of Error
Although it is possible to determine if a suspected false-positive
culture result is confirmed, likely, or unlikely without first identifying
the precise mechanism that led to the problem, it is obviously important
to document this mechanism so that it can be corrected. Identifying
the precise nature of the problem also aids in our understanding
of how these types of errors can occur and the importance of adhering
to procedures that will prevent them.
The laboratory or health-care facility that was involved should
be contacted and provided the results of the investigation and the
preliminary interpretation. In a collaborative fashion, the investigation
should be finalized, and steps should be taken to describe as thoroughly
as possible the precise mechanism that led to the false-positive
culture result. Necessary procedural changes should be described
and instituted, including updated quality-control and quality-assurance
procedures. Technical assistance may be required and should be offered
by state and national reference laboratories.
Communicating Results of Investigation
Once a final determination is made about the likelihood of a culture
result being false and the likely source of error, the TB program
should communicate the results of the investigation to the appropriate
persons. The health-care providers of the patient or patients who
were misdiagnosed need to receive this information immediately.
The facility or laboratory that was determined to be the source
of the error should also receive the final report of the investigation.
Finally, there should be a formal reporting process for collecting
and analyzing the results of false-positive cultures so that TB
programs can monitor their frequency and track problems that can
Possible Additional Steps
- Send a fact sheet to each implicated laboratory during the
investigation describing the risk of M. tuberculosis laboratory
cross-contamination and steps to prevent it.
- Assess and educate clinical laboratories when a false-positive
culture event is identified.
- Provide local and statewide education to laboratory staff regarding
prevention steps and early detection of persons who have false-positive
- Provide local and statewide education to local program staff
regarding early detection of laboratory cross-contamination.
- Report the number of false-positive events, number of persons
treated, number of months treatment was given, and basic characteristics
of laboratories where these events occurred.
- Summarize these data on a national level for use in recommendations
for mycobacteriology laboratory practice.
- Review findings periodically to determine whether specimen
contamination has been reduced or whether contamination is suspected