The BACTEC TB system in the laboratory diagnosis of mycobacterial infection

to compare the BACTEC TB system with our conventional multimedia system for the isolation of mycobacteria from clinical specimens.

specimens received over a two year period were cultured into BACTEC 12B vials and onto a variety of solid and liquid media routinely used by us for the isolation of mycobacteria. Cultures were observed regularly for bacterial growth, and examined for the presence of mycobacteria by acid fast microscopy. All results were recorded. At the end of the two year period the results for the two methods were compared.

of the 4443 specimens cultured by both methods, 260 were positive for mycobacteria. Sensitivity and specificity were 88.31% and 99.35% respectively for the BACTEC, and 84.41% and 99.96% respectively for our routine multimedia system. Tuberculosis was confirmed by the BACTEC TB system in six patients who were otherwise culture negative. One of these patients had tuberculous meningitis. Three patients culture positive for M tuberculosis by our routine method, were negative by the BACTEC. Twenty-nine false positive cultures were detected, 27 by the BACTEC TB system alone.

the BACTEC TB system is a rapid and sensitive system for the detection of mycobacteria in clinical specimens, and has proved to be a useful adjunct to our routine multimedia system. However, decreased specificity, particularly with lower respiratory tract specimens, means that positive results from these sites can be reported less confidently when they are detected by the BACTEC TB system alone. This negates some of the advantages of the BACTEC TB system for the detection of mycobacterial disease of the lungs.