MycobacteriologyMulticenter evaluation of two commercial amplification kits (Amplicor, Roche and LCx, Abbott) for direct detection of Mycobacterium tuberculosis in pulmonary and extrapulmonary specimens
Introduction
Tuberculosis is re-emerging as a major problem, not only in the third world, but also in developed countries, where numerous community outbreaks have been reported (Raviglione et al. 1995). Furthermore the presence of multiple drug resistance in several strains of Mycobacterium tuberculosis (Perronne 1993) can greatly worsen the outcome of the disease, with an increase of fatality ratios. A prompt diagnosis of new cases is a prerequisite for the prevention of secondary cases (Tenover et al. 1993).
The availability of commercial amplification kits made easier the introduction of molecular diagnostic procedures in mycobacteriology laboratories. Sensitivity and specificity of such commercial assays have been thoroughly assessed, but little has been done to compare them directly. LCx M. tuberculosis (Abbott Diagnostic Division, Abbott Park, Ill, USA) is the most recent commercial amplification kit, and we carried out a multicenter assessment of its performance, in direct comparison with the well tested Beavis et al 1995, Bergmann and Woods 1996, Carpentier et al 1995, Cartuyvels et al 1996, D’Amato 1995, D’Amato 1996, Devallois et al 1996, Huang et al 1996, Ichiyama et al 1996, Moor and Curry 1995, Piersimoni et al 1997, Schirm et al 1995, Soini et al 1996, Wobeser et al 1996 Amplicor M. tuberculosis assay (Roche Diagnostic System, Branchburg, NJ, USA).
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Clinical specimens
The comparison was based on 697 clinical specimens from 481 patients, and it was performed in three different laboratories (Table 1). Whereas laboratory B included into the comparison 303 consecutively received samples, laboratories A and C limited the analysis to about 50% of specimens processed during the experimentation period, which lasted 3 months. A volume of the sample lower than 5 mL and the provenance from divisions known for their low prevalence of M. tuberculosis isolations were the
Results
One hundred thirty-five specimens grew mycobacteria in culture, 110 of which were identified as M. tuberculosis; of the latter, 84 came from microscopy-positive samples, 26 from negative ones. The 25 nontuberculous strains included eight Mycobacterium avium, eight Mycobacterium xenopi, five Mycobacterium kansasii, three Mycobacterium gordonae, and one Mycobacterium celatum.
The two amplification methods yielded very close values of sensitivity and specificity in comparison with culture (Table 3)
Discussion
Usually the evaluations of amplification systems resort to final resolution, on the basis of clinical diagnosis of tuberculosis, only for cases characterized by results disagreeing with the culture. In this study a further source of discrepancies is represented by the parallel use of two biomolecular methods that revealed a number of disagreements between the amplification results. While, limiting the use of resolution to discrepancies among amplification and culture, Amplicor and LCx seem
Acknowledgements
We thank P. Urbano (Institute of Microbiology, University of Florence, Italy) for valuable advice.
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