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Journal of Clinical Microbiology, January 2001, p. 69-74, Vol. 39, No. 1
0095-1137/01/$04.00+0   DOI: 10.1128/JCM.39.1.69-74.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Rapid Identification of Laboratory Contamination with Mycobacterium tuberculosis Using Variable Number Tandem Repeat Analysis

Deborah M. Gascoyne-Binzi,1,2,* Rachael E. L. Barlow,2 Richard Frothingham,3 Grant Robinson,1 Timothy A. Collyns,1 Ruth Gelletlie,4 and Peter M. Hawkey1,2

Department of Microbiology, The General Infirmary, Leeds LS1 3EX,1 Division of Microbiology, The University of Leeds, Leeds LS2 9JT,2 and Bradford Health Authority, Bradford BD18 3LD,4 United Kingdom, and Durham VA Medical Center, Durham, North Carolina 277053

Received 22 June 2000/Returned for modification 3 September 2000/Accepted 20 October 2000

Compared with solid media, broth-based mycobacterial culture systems have increased sensitivity but also have higher false-positive rates due to cross-contamination. Systematic strain typing is rarely undertaken because the techniques are technically demanding and the data are difficult to organize. Variable number tandem repeat (VNTR) analysis by PCR is rapid and reproducible. The digital profile is easily manipulated in a database. We undertook a retrospective study of Mycobacterium tuberculosis isolates collected over an 18-month period following the introduction of the BACTEC MGIT 960 system. VNTR allele profiles were determined with early positive broth cultures and entered into a database with the specimen processing date and other specimen data. We found 36 distinct VNTR profiles in cultures from 144 patients. Three common VNTR profiles accounted for 45% of true-positive cases. By combining VNTR results with specimen data, we identified nine cross-contamination incidents, six of which were previously unsuspected. These nine incidents resulted in 34 false-positive cultures for 29 patients. False-positive cultures were identified for three patients who had previously been culture positive for tuberculosis and were receiving treatment. Identification of cross-contamination incidents requires careful documentation of specimen data and good communication between clinical and laboratory staff. Automated broth culture systems should be supplemented with molecular analysis to identify cross-contamination events. VNTR analysis is reproducible and provides timely results when applied to early positive broth cultures. This method should ensure that patients are not placed on unnecessary tuberculosis therapy or that cases are not falsely identified as treatment failures. In addition, areas where existing procedures may be improved can be identified.

* Corresponding author. Mailing address: Department of Microbiology, The General Infirmary, Great George St., Leeds LS1 3EX, United Kingdom. Phone: 44 113 392 3929. Fax: 44 113 233 5649. E-mail: deborahg{at}pathology.leeds.ac.uk.

Journal of Clinical Microbiology, January 2001, p. 69-74, Vol. 39, No. 1
0095-1137/01/$04.00+0   DOI: 10.1128/JCM.39.1.69-74.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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