European Society of Mycobacteriology - Instituto Nacional de Saúde ...

PP-74
PRACTICAL EXPERIENCE OF USING A DNA AMPLIFICATION
ASSAY FOR RAPID DETECTION OF Mycobacterium
tuberculosis COMPLEX IN RESPIRATORY SPECIMENS
J. Cacho 1 , A. García-Cañas 1 , A. González Torralba 1 , I. Cano 2 , A. Pérez Meixeira 3 , A. Ramos Martos 2 , and M. Sánchez-Concheiro 1
1 - Servicio de Microbiología, Hospital Universitario de Getafe, Madrid
2 - Servicio de Neumología, Hospital Universitario de Getafe, Madrid
3 - Servicio de Salud Pública, Comunidad de Madrid, Madrid.
Objectives
To evaluate experience in a clinical microbiology laboratory of using a DNA amplification assay for routine detection
of Mycobacterium tuberculosis complex (MTC) performed once weekly, and to compare this method with microscopy
and culture.
Methods
A total of 507 respiratory specimens from 419 patients were screened for tuberculosis (TB). Smear examinations, culture
and polymerase chain reaction (PCR) were performed on each sample. Specimens were processed according to standard
laboratory protocols. All samples were processed exactly as described in Cobas Amplicor MTB Methods Manual (Roche
Molecular Systems, USA). This method was performed once per week.
The following two groups of samples were considered to be true positive: (1) samples which were culture positive for
MTC; and (2) all samples which were culture negative for MTC but positive to PCR, provided that one or more of the
following criteria were met: (i) the samples originated from a patient whose other samples were culture positive; (ii) the
patient’s clinical history provided evidence of TB sufficient to warrant initiating treatment for TB.
Results
Inhibition of PCR was seen in 15 (2.9%) specimens. A total of 37 (7.3%) samples were considered to be true positive:
33 samples grew MTC in culture and 4 were culture negative for MTC but positive for PCR and met the criteria as described
previously. The overall sensitivity and specificity of PCR as compared to true positive samples was 83.8% (31/37)
and 99.1% (466/470), respectively; that of culture 89.2% (33/37) and 100% (470/470), respectively; and that of direct
microscopy 64.9% (24/37) and 99.8% (469/470), respectively.
The average time to reporting for true positive samples was 6 days for positive PCR and 11.4 days for positive culture. Of
the smear-positive, true positive samples, 91.7% (22/24) were PCR positive. Of the smear-negative, true positive samples,
69.2% (9/13) were PCR positive. The 9 samples classified as true positive and smear negative were from 9 different patients.
Treatment was started earlier in 44.5% of these patients because positive PCR findings were obtained. The average
time to reporting was 5.4 days for positive PCR findings and 17.4 days for positive culture findings.
Conclusion
MTC infection was confirmed for PCR in 91.7% of smear-positive specimens. Although PCR was performed once weekly,
treatment in 44.5% of patients was initiated earlier because of positive PCR results from smear-negative samples.
146 ESM 2009