Question of the Week

Question :
Posted On : 05 Apr 2016
what are the newer tests for tuberculosis like DNA PCR, adesonine deaminase,other haematological tests
Expert Answer :
Older techniques of TB culture such as inoculation on LJ medium for 6 weeks have now been replaced by better and faster molecular techniques. Culture can detect 100 bacilli, ml of sputum in comparison to more than 10,000 bacilli, ml needed for microscopy. Also culture provides material to identify atypical mycobacteria from M. TB and also is useful for drug sensitivity testing. The newer culture techniques are Bactec radiometric system. Bactec system was a breakthrough for diagnosis of M. TB as it allowed isolation in a few days {within 14 days} as compared to culture media. Polymerase chain reactions {PCR} are now increasingly used for diagnosis of TB. Species specific DNA probes that hybridize to rRNA for the identification of several important mycobacteria including M. TB, M. avium, M. intracellulare, M. avium complex, M. Kanasii and M. gorelonae are available and have good sensitivity and specificity. When done on culture positive specimens can help to identify type of mycobacterium within 2 hours. When PCR is done on tissue specimen directly, it cannot differentiate between dead and live bacillus and thus may give false positive results. False negative results may be seen if amplification reaction inhibitors are present in the specimen. Other tests are Interferon Gamma assays. ADA is an enzyme present in most cells but is increased in tubercular serositis. Thus increased ADA levels in pleural effusions, ascitis and pericardial effusions may suggest TB and can be an indirect marker for detection of TB serositis. CSF ADA levels are not good marker for diagnosis of TBM. Thus ADA may be an additional tool for diagnosis of TB serositis.
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