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3 Dec 2013

TB DIAGNOSIS—A NEW HORIZON

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Rapid diagnosis of tuberculosis and resistance profile is central to its control. Failure to quickly recognize and treat affected patients leads to increased mortality, secondary resistance (including extensively drug-resistant tuberculosis), and ongoing transmission.

A fully automated molecular test for TB case detection and drug resistance (rifampicin resistance) has been recently developed by the name of Xpert MTB/RIF which integrates sample processing and PCR in a disposable plastic cartridge containing all reagents required for bacterial lysis, nucleic acid extraction, amplification, and amplicon detection.
Processing involves three simple steps:

  1. Sputum liquefaction and inactivation with sample reagent (15 minutes)
  2. Transfer of material (from step 1) into test cartridge
  3. Insertion of cartridge into machine (Xpert MTB/RIF)

Subsequent processing is automated, and the results are provided in 1 hour and 45 minutes (total time from sputum processing to result: 2 hours). The results are provided in a printable version e.g. MTB detected; RIF resistance not detected.

Case detection:
Sensitivity: 99.8% in smear & culture positive and 90.2% in smear negative & culture positive (overall 97.6%)
Specificity: 99.2% overall

Rifampin resistance detection:
Sensitivity: 99.1%
Specificity: 100%

Advantages:
Point-of-treatment use, results available in the clinic and decision regarding therapy could be taken
No requirement of a biological safety cabinet
Minimum training required for test performer (2-3 days v/s 2 weeks for microscopy)
Simple to perform, minimum hands-on-time of 15 minutes
Unambiguous result readout

Disadvantages
Cost of the equipment
Needs a reliable electricity supply and temperature maintenance
Requirement of annual calibration

Reference:
Boehme CCN, P; Hillemann, D; Nicol, M.P; Shenai, S; Krapp, F. Rapid Molecular Detection of Tuberculosis and Rifampin Resistance. New Engl J Med. 2010.

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