The effect of changes in laboratory practices on the rate of false-positive cultures for Mycobacterium tuberculosis.

CONTEXT False-positive cultures for Mycobacterium tuberculosis have been found in nearly all DNA fingerprinting studies, but the effectiveness of interventions to reduce cross-contamination has not been evaluated. OBJECTIVE To evaluate whether changes in laboratory policies and procedures reduced the rate of false-positive cultures. DESIGN Retrospective study of isolates with matching DNA fingerprints. SETTING A mycobacteriology laboratory serving an urban tuberculosis control program and public hospital system. PATIENTS All M tuberculosis isolates processed from July 1994 to December 1999. METHODS Isolates were fingerprinted using IS6110; pTBN12 was used to fingerprint isolates having fewer than 6 copies of IS6110. We further evaluated all patients having only one positive culture whose DNA fingerprint matched that of another isolate processed in the laboratory within 42 days. INTERVENTIONS We changed laboratory policy to reduce the number of smear-positive specimens processed and changed laboratory procedures to minimize the risk of cross-contamination during batch processing. MAIN OUTCOME MEASURE The rate of false-positive cultures. RESULTS Of 13 940 specimens processed during the study period, 630 (4.5%) from 184 patients and 48 laboratory proficiency specimens grew M tuberculosis. There were no cases (0/184) of probable or definite cross-contamination, compared with the 4% rate (8/199) identified in our previous study (P =.008). We also fingerprinted a convenience sample of isolates from other laboratories in Denver; 13.6% (3/22) of these were false-positive, a rate similar to the 11.9% rate (5/42) identified for other laboratories in our previous study (P =.84). CONCLUSIONS Laboratory cross-contamination decreased significantly after relatively simple, inexpensive changes in laboratory policies and practices. Cross-contamination continued to occur in other laboratories in Denver.