Mycobacteriology
Multicenter evaluation of two commercial amplification kits (Amplicor, Roche and LCx, Abbott) for direct detection of Mycobacterium tuberculosis in pulmonary and extrapulmonary specimens

https://doi.org/10.1016/S0732-8893(98)00097-2Get rights and content

Abstract

Direct detection of Mycobacterium tuberculosis was performed in parallel with the Amplicor M. tuberculosis test (Roche Diagnostic System, USA) and the LCx M. tuberculosis (Abbott Diagnostic Division, USA) on 697 samples, collected from 481 patients, in three different Italian laboratories. Though both systems are licensed only for pulmonary specimens, 113 extrapulmonary specimens (represented mainly by pleural fluids, cerebrospinal fluids and urines) were included in the study. Amplification results were compared with acid-fast microscopy, culture, and identification of isolates. Final clinical diagnosis was used to resolve discrepant results. M. tuberculosis was detected in 105 specimens by both assays, whereas 561 were agreeing negatives; 21 and 6 of the remaining true-positive samples scored positive with LCx only and with Amplicor only, respectively. There were three false-positives with LCx and one false-positive with Amplicor. The diagnostic sensitivity of both methods was significantly better when only respiratory specimens were considered (78% versus 59% in nonrespiratory samples with Amplicor, and 88% versus 65% with LCx). Our data reveal a significantly better sensitivity of the LCx (p = 0.026) and a slight better specificity of the Amplicor assay. It is noteworthy that 16 of the 21 Amplicor-negative specimens in which LCx detected M. tuberculosis were culture negative, thus suggesting that the higher diagnostic sensitivity of the latter assay is attributable to its better analytical sensitivity. However, the majority of such samples originated from patients under antimicrobial treatment, which makes uncertain the clinical significance of such increased sensitivity. Considering true-positive for LCx and true-negative for Amplicor, the 16 culture-negative/LCx-positive/Amplicor-negative specimens resulted true-positives after the resolution of discrepancies, the final overall sensitivity and specificity values of the LCx assay were not significantly different from the ones of the Amplicor assay.

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).

Section snippets

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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