Residual DNA of Mtb from the bronchoscope channel as a possible cause of false positive results in the diagnosis of tuberculosis

The use of bronchoalveolar lavage (BAL) as a diagnostic material increases the efficiency of TB diagnostics, but there remains the possibility of obtaining false positive results due to the presence of residual Mtb DNA in the bronchoscope channel. Aim: to examine bronchoscopes after high-level disinfection to determine whether residual Mtb DNA. Materials and methods: The 42 samples were taking from video bronchoscopes OLYMPUS-160 and PENTAX -1975K after high-level disinfection. The inner channel of the bronchoscope was additionally washing with 40 ml of sterile saline. The resulting sample was centrifuged at 3000g for 15 min. Then 0.5 ml of the precipitate was tested by PCR-RT for detection of Mtb DNA. The sample was considering positive for the presence of a specific Mtb DNA fragment IS6110, if the threshold cycle value was less than 45. Results: DNA fragments of Mtb IS6110 were founding in six samples, which accounted for 14.3% of all examined samples. The value of the threshold cycles was 32.0, 33.8, 37.8, 38.0, 40.3 and 41.0 respectively. It should be noting that none of the negative controls included in the sample preparation or amplification step of the assay were positive. Conclusions: The results of the study showed that residual Mtb DNA  remained in clean bronchoscopes in 14.3% of cases, even after high level disinfection, and  potentially be a cause of false-positive PCR results in subsequent diagnostic samples. Therefore, it is necessary to interpret the results of BAL fluid PCR for the diagnosis of TB with great caution, taking into account the clinical and radiological picture and the results of other methods of examining the patient.