Background: Mycobacterial false-positive cultures have rarely been recognized in Korea, even though the rate of false-positive cultures of Mycobaterium tuberculosis has ranged from 0.4% to 4.0%. We estimated the false-positive rates by the review of medical records from whom mycobacterial cultures were requested, retrospeaively, after a bout of false-positive cultures was discovered in specimens treated in a single day.

Methods : Of the total 2,245 specimens, including 337 positive cultures of mycobacteria, during the period of January and June 1999, seventy-two specimens that showed colonies less than or equal to 5 colonies were reviewed, and classified as tuberculosis-likely group, tuberculosis-unlikely group and unclassifiable group by the clinical and radiological evidences, anti-tuberculosis therapy, and microbiological results.

Results : Tuberculosis-unlikely group was 21 specimens from 20 patients, and unclassifiable group was five specimens from four patients. So, the false-positive rates were estimated as 0.91.1% of total cultures and 6.2-7.7% of positive cultures, according to excluding or including the unclassifiable group.

Conclusion: Care should be taken for lowering false-positive mycobacterial cultures. Especially when a culture turned out to be positive with low colony isolates, more careful interpretations should be preceded before reporting the results by the review of medical records and communication with physician in charge. (Korean J Clin Microbiol 2001;4:40-44)