A patient with suspected pulmonary TB in India is evaluated for INH and rifampicin resistance. GeneXpert MTB/RIF Ultra is performed on sputum. The test detects rifampicin resistance by:

• A Culture on LJ medium with drug incorporated and growth comparison
• B Molecular beacons targeting katG and inhA promoter mutations
• C MODS (microscopic observation drug susceptibility) method
• D Real-time PCR probes targeting 81-bp hot-spot region of rpoB gene for mutations ✓

Explanation

GeneXpert MTB/RIF (and Ultra version) uses semi-nested real-time PCR with molecular beacon probes targeting the 81-bp 'rifampicin resistance-determining region' (RRDR) of the rpoB gene; mutations in this region account for >96% of rifampicin-resistant TB. katG and inhA mutations are detected by LPA (GenoType MTBDRplus) for isoniazid resistance. LJ slant culture with drug is the phenotypic DST gold standard. MODS is a low-cost liquid culture microscopic method.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.