TB Diagnosis a new Horizon

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:

Sputum liquefaction and inactivation with sample reagent (15 minutes)
Transfer of material (from step 1) into test cartridge
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.

Imran Haq