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Guide to the Application of Genotyping to Tuberculosis Prevention and Control										Related Pages
										**Tuberculosis Genotyping Case Studies: How TB Programs Have Used Genotyping**
**False-Positive Culture Investigation**
In early 2003, a state mycobacteriology laboratory began testing the Mycobacterium Growth Indicator Tube (MGIT) 960 system as a potential replacement for their current BACTEC 460 system for culture of *M. tuberculosis* in broth media. The MGIT 960 is an automated culture system that automatically checks culture tubes for growth every hour and does not depend on a technician reviewing culture results twice each week, as was required by the former system. During February and March 2003, the BACTEC and the MGIT systems were used in parallel.
In March 2003, several *M. tuberculosis* isolates cultured in the state laboratory were reported to be resistant to isoniazid (INH) and streptomycin. In April 2003, several isolates also demonstrated resistance to isoniazid (INH) and ethambutol. Since these susceptibility patterns are unusual, the concern for possible false-positive culture results was raised and a formal investigation was initiated.
Spoligotyping and MIRU analysis identified three separate genotyping clusters during this time period. The first cluster consisted of two isolates with isoniazid and streptomycin resistance. One of the isolates came from a patient with three positive cultures and an abnormal chest x-ray; the other came from a patient with only one positive culture and a history of blunt trauma to the chest. This patient’s specimen was processed 1 day after the first patient’s specimen was processed for drug susceptibility testing.
The second cluster involved isolates from six patients, each with only one positive culture. Only one patient had a clinical picture suggestive of tuberculosis, and this patient was in his 11th month of anti-TB treatment administered by directly observed therapy. The spoligotyping and MIRU results from these six isolates matched a quality-control strain used by the laboratory. In each instance, patient specimens were processed for drug susceptibility testing within 1-2 days of the quality-control strain.
The third cluster involved only one patient. This patient had only one positive culture result and had a clinical picture that was not particularly suggestive of tuberculosis disease. This patient’s isolate matched the genotyping pattern of a proficiency strain the laboratory processed 1 day before during drug susceptibility testing.
A review of laboratory procedures revealed that new laboratory protocols were required for use of the MGIT system. These protocols stipulated that a manual micropipettor should be used to inoculate tubes for drug susceptibility testing and for adding sterile supplement to broth tubes for culture of new specimens. Additional micropipettors had been ordered, but they had not arrived yet at the time of the contamination. Therefore, the same manual micropipettor was used to inoculate tubes for DST each afternoon and to add sterile supplement to broth tubes each morning. These broth tubes were subsequently inoculated with incoming specimens/isolates via sterile, disposable pipettes.
This investigation highlights several common findings when false-positive cultures occur. All of the patients with false-positive cultures had only one positive culture result (all the true cases had more than one positive culture result). All of the patients with false-positive culture results did not have clinical pictures that were particularly suggestive of active tuberculosis, and in all cases, the contaminated cultures and the cultures that were the sources of the contamination were processed within 1-2 days of each other. Genotyping helped the laboratory staff and clinicians communicate rapidly and terminate unnecessary treatment.
As you will see when we discuss algorithms for identifying possible false-positive culture results, each of these findings are red flags and should be evaluated each time a genotyping cluster is reported.
							
					
						Page last reviewed:  September 1, 2012					Content source: 	Division of Tuberculosis Elimination, National Center for HIV, Viral Hepatitis, STD, and TB Prevention, Centers for Disease Control and Prevention homeGuide to the Application of Genotyping to Tuberculosis Prevention and Control
Acknowledgments
Introduction to Tuberculosis Genotypingplus icon
Introduction to this Guide
Overview of Tuberculosis Genotyping
Overview of the CDC Tuberculosis Genotyping Program
TB Program Eligibility and Application Procedures
Tuberculosis Genotyping Case Studiesplus icon
Investigation of a Genotyping Cluster in a Low-Incidence State
Apparent Genotyping Cluster among Recent Immigrants from the Philippines
False-Positive Culture Investigation
Algorithm to Detect False-Positive Cultures
A Misdiagnosis that was Identified with Genotyping
Genotyping Used to Improve Contact and Cluster Investigations
Identification of Nontraditional Transmission Settings
Investigation of a Multistate Tuberculosis Outbreak
Homeless Shelter Outbreak Investigation
Using Genotyping to Evaluate the Impact of a Screening Program
CDC Tuberculosis Genotyping Laboratory Proceduresplus icon
Science behind Tuberculosis Genotyping
Description of Genotyping Methods
Submission of Isolates
Genotyping Results, Designation of Clusters, and Reporting
Suspected False-Positive Cultures
IS*6110*-based RFLP Analysis
Combining Genotyping and Epidemiologic Data to Improve Our Understanding of TB Transmissionplus icon
Definitions
Reading the Genotyping Laboratory Report
Comparing Genotyping and Epidemiologic Results from Two TB Patients
Comparing Genotyping and Epidemiologic Results from Three or More TB Patients
Clustering as a Surrogate Measure of Recent Transmission
Developing a Tuberculosis Genotyping Programplus icon
Developing a Written Plan
Initial Decisions
Establishing Procedures for:plus icon
Submitting Isolates
Receiving and Acting on Genotyping Results
Requesting RFLP Analysis
Submitting Additional Isolates from the Same Patient
Comparing Genotyping Results between Jurisdictions
False-Positive Cultures
Database Management and Quality Control
Special Issues
Applying Genotyping Results to Tuberculosis Control Practicesplus icon
Evaluating Matching Genotypes
Evaluating Nonmatching Genotypes
Suspected False-Positive Culture Investigations
Expanded Contact Investigations
Outbreak Investigations
Cluster Investigations
Monitoring Trends
Appendicesplus icon
Appendix A: Glossary and Abbreviations
Appendix B: References and Useful Resourcesplus icon
References Cited in the Guide
NTGSN Special Issue References
Information on Packaging and Shipping Infectious Substances
Core Steps of an Outbreak Investigation
Consultants Available to Offer Guidance To receive email updates about this page, enter your email address: Email Address			What's this?				Submit


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