Guide to the Application of Genotyping to Tuberculosis Prevention and Control

Tuberculosis Genotyping Case Studies: How TB Programs Have Used Genotyping

Algorithm to Detect False-Positive Cultures

Genotyping can help identify instances of incorrect TB diagnoses that are based on false-positive cultures. Incorrect diagnoses can result from laboratory cross-contamination of cultures, mislabeling of patient specimens, collection errors, and reporting errors. Such errors occur in an estimated 1%–3% of all reported cases of TB, and as many as 300 persons per year in the United States may be started erroneously on anti-tuberculosis treatment. Universal genotyping permits TB programs to establish simple algorithms to flag suspected errors.

The California Department of Health Services, in collaboration with San Francisco General Hospital, Santa Clara Valley Medical Center, and Solano County Public Health Laboratory, evaluated this type of algorithm to find methods to decrease the occurrence of laboratory cross-contamination (Jasmer 2002).

During the study period from January 1998 through June 1999, cultures for 296 patients were positive for M. tuberculosis, and cultures for ten patients met one or more of the criteria for possible false-positive cultures. A review by a panel of experts determined that TB was misdiagnosed for six patients (representing 2% of all patients with cultures positive for M. tuberculosis) because of laboratory cross-contamination. In four of the six cases, contamination probably occurred when reagents were dispensed from a common flask. This practice was discontinued on the basis of the findings of this study. (See Chapter 6, Applying Genotyping Results to Tuberculosis Control Practices, for more information on investigating false-positive cultures.)