Evaluation of low-colony-number counts of Mycobacterium tuberculosis on solid media as a microbiological marker of cross-contamination.

Low-colony-number counts on solid media are considered characteristic of cross-contamination, although they are normally observed in true-positive cultures from some groups of patients. The aim of this study was to evaluate low-yield growth cultures as a microbiological marker for cross-contamination. We evaluated 106 cultures with <15 colonies from 94 patients, and the proportions of false-positive cultures were 0.9% per sample and 1.1% per patient, which indicates that low-yield growth is not a reliable marker of cross-contamination.