A retrospective observational study was performed to determine the sensitivity and limitation of PCR test for the detection of Mycobacterium tuberculosis and M. avium complex. We obtained clinical specimens collected from the respiratory tract, cultured M. tuberculosis or M. avium complex, and performed PCR analysis. A total of 299 samples (M. tuberculosis, 177; M. avium, 35; M. intracellulare, 87) were analyzed by COBAS TaqMan PCR from April 2007 to March 2011. The PCR positivity rates were 50–55%, 70–100%, 88–98%, and 100% in smear-negative, smear 1+, 2+, and 3+ groups, respectively. The PCR positivity of tuberculosis in smear 1+ was 80.6%, which was statistically significantly ( ) lower than that of smear 2+ (97.3%). From January 2005 to March 2007, we collected an additional 138 samples (M. tuberculosis, 74; M. avium, 21; M. intracellulare, 43), which were analyzed by COBAS Amplicor PCR. The PCR positivity rates obtained using COBAS TaqMan PCR and COBAS Amplicor PCR were not significantly different. The sensitivity of PCR test for mycobacteria is not sufficient in case of smear 1+. Careful consideration must be given to the interpretation of negative PCR test results in smear 1+, because smear-positive tuberculosis is the criterion for isolation. 1. Introduction Polymerase chain reaction (PCR), which was invented by Kary Banks Mullis [1, 2], is widely used in basic and clinical medicine. Particularly in the field of clinical medicine, PCR plays an important role in the early diagnosis of infectious diseases [3], because PCR can detect as little as one copy of DNA fragment from a pathogenic organism. The utility of PCR has also been reported in the diagnosis of mycobacteriosis [4, 5], and PCR is widely used for the detection of Mycobacterium tuberculosis and M. avium complex (MAC) in Japan. On the other hand, a specimen with a positive mycobacterial smear and a negative M. tuberculosis PCR test may sometimes exhibit a positive M. tuberculosis culture result. Such false negative results are very dangerous, because they may lead to the release of patients that pose an infection control risk. We evaluated the reliability and limitation of PCR test for the detection of M. tuberculosis, M. avium, and M. intracellulare in relation to the clinical situation. We found out that PCR result is sometimes not reliable in smear 1+ case. 2. Methods 2.1. Study Subjects Clinical specimens collected from the respiratory tract (sputum or samples obtained using a bronchofiberscope (BF)) were analyzed from January 2005 to March 2011. M. tuberculosis, M. avium, or M.
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