RAPID DIAGNOSTIC METHODS1

CTRI of the Russian Academy of Medical Sciences uses both conventional and newer, rapid methods of mycobacteria identification (see Box 5-1 for an overview of current TB diagnostic methods). The conventional methods used are fluorescent microscopy and culture, which can require up to 10 weeks for culturing and an additional 4 weeks for drug susceptibility testing. During this time, physicians and patients must wait to determine how to treat the patient’s TB, and this delay provides an opportunity for the disease to spread. To shorten this time, CTRI has been using commercial products that rely on culturing and on molecular-genetic methods. These products can determine which strains are resistant or sensitive to specific drugs through colorimetric methods, with automatic detection and no use of test tubes, and reduce the time required to obtain drug susceptibility results to 6−13 days.

The molecular-genetic methods rely on detection of mutations in the DNA of M.tb. that convey drug resistance. One such method uses biochips developed in Russia (see the next section). It involves the extraction of M.tb. DNA, two-stage polymerase chain reaction (PCR), hybridization with amplicons labeled with fluorescent marks on the biochip, and detection of results using an analyzer with subsequent computer processing.

A comparison of biochip and culture data showed a concordance of 95 percent for rifampicin resistance, 88.5 percent for isoniazid resistance, and 87 percent for fluoroquinolone resistance. These are reassuring numbers, said Larionova, since not all mutations responsible for resistance are found using biochips.

Other molecular-genetic methods used to detect drug resistance involve DNA-strip or related technologies. They rely on DNA extraction, amplification by PCR, hybridization on strips, and visualization and estimation of results. These methods are highly safe, easy to use, and cost-effective. Results are available within 1−2 days and can be obtained from either solid or liquid media. A comparison of results from biochips and DNA strips demonstrated full concordance.

The above technologies allow for the detection of MDR and XDR TB during a patient’s examination and the administration of adequate chemotherapy regimens to shorten the sputum conversion period, improve treatment outcomes, and prevent the spread of disease. Experience at CTRI indicates that 64 percent of new MDR TB cases convert after 2 months of treatment and 87 percent after 6 months of treatment.

New methods are also being used to diagnose infection with nontuber-

 

 

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1 This section is based on the presentation of Elena Larionova, Central TB Research Institute, Russian Academy of Medical Sciences.



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