False-positive TB cases: Much ado about nothing

False-positive cultures for Mycobacterium tuberculosis due to laboratory error can result in misdiagnosis of patients, unnecessary administration of TB medications, and considerable time and labor to track down and test potential contacts, the Centers for Disease Control and Prevention reminds.1

When TB culture results are inconsistent with the patient’s signs and symptoms or clinical course, clinicians should discuss the case with the laboratory and the local or state health department, the CDC advises.

The CDC emphasized the issue in light of five such incidents that resulted in the potential misdiagnosis of TB in 11 people in Wisconsin last year. Four incidents were associated with cross-contamination of specimens in the laboratory, and one was traced to inadvertent inoculation of a subculture from the incorrect broth medium. Before recognition that these cultures were false-positive, 10 patients or their families had been informed of the diagnosis of TB, and eight patients received unnecessary medical treatments. One patient was hospitalized in respiratory isolation, two were administered bronchos copies, and seven were placed on anti-TB medications. Ten of these false-positive cases were reported to local health departments, spurring contact tracing that resulted in 108 family and social contacts receiving TB skin tests. All were negative. In addition, 328 hospital employees and patients received skin tests, and nine had chest radiographs. No evidence of transmission was found.

"Hospital infection control and employee health staff expended an additional estimated 330 person-hours as a result of these episodes," the CDC reports.

These findings in Wisconsin are similar to those in other recent reports documenting the occurrence of false-positives ranging from 1.2% to 4.0%. False-positive results may be even more common in outbreak situations. Based on a review of records for 223 TB culture-positive patients in outbreaks in five states, the clinical course was inconsistent with TB in 26% of patients, the CDC reports.

Indicators of potential false-positive TB cultures include:

• All specimens from a patient are AFB smear-negative, and only one is M. tuberculosis culture-positive.

• The patient’s signs, symptoms, and clinical course are inconsistent with TB.

• An M. tuberculosis culture-positive specimen, also likely to be AFB smear-positive, was processed the same day as the suspected specimen.

• The DNA fingerprint pattern of the suspected isolate is identical to that of the putative source isolate.

• There are no known epidemiologic links between the patient with the suspected isolate and the patient with the putative source isolate.

• The duration of time for detection of growth in the suspected culture was prolonged, or only sparse colonies were detected on solid medium.


1. Centers for Disease Control and Prevention. Multiple misdiagnoses of tuberculosis resulting from laboratory error — Wisconsin, 1996. MMWR 1997; 46:797-801.