Molecular Assay Employed for Extrapulmonary TB Specimens

The laboratory diagnosis of tuberculosis (TB) on extrapulmonary specimens is particularly challenging, as invasive procedures are often required in order to obtain samples for culture.

A number of commercial nucleic acid amplification tests able to detect and identify Mycobacterium tuberculosis (MTB) complex directly from respiratory secretions have been developed, but their use on extrapulmonary samples still calls for validation.

Scientists at the United Hospitals (Ancona, Italy) performed a retrospective analysis aiming to evaluate the diagnostic accuracy of a Complex Direct Detection Assay (DTB) on 918 nonrespiratory specimens collected from 863 patients investigated for TB based on different levels of clinical suspicion, between January 2006 and December 2009. The specimens included 84 gastric aspirates, 145 urine, 136 sterile body fluids, 83 cerebrospinal (CSF) fluids, 237 fine-needle aspirates, 175 pus, 56 biopsies, and 2 stool specimens.

The results were compared with those of acid-fast staining and culture, solid plus liquid media, setting the combination of culture and clinical diagnosis as the gold standard. Ninety-two specimens yielded culture positive for MTB and 24 who were smear and culture negative were from patients with TB clinical diagnosis. Of these patients, 96 were positive with the BDProbeTec ET Mycobacterium tuberculosis Complex Direct Detection Assay (DTB, Becton, Dickinson and Company, Franklin Lakes, NJ, USA) including all of those from culture-negative TB cases. From 26 specimens, nontuberculous mycobacteria were grown and two of these specimens were positive by the DTB assay.

The authors concluded that the overall sensitivity, specificity, and positive and negative predictive values of the DTB assay for extrapulmonary TB were high. A reduced sensitivity of 73.5% was observed among smear-negative specimens when compared with 91.7% for smear-positives specimens. Although amplification assays cannot replace culture techniques, DTB proved to be rapid and specific for the detection of MTB in extrapulmonary samples. The study was published in the March 2012 issue of the European Journal of Clinical Microbiology & Infectious Diseases.

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