GeneXpert MTB/RIF Ultra (Xpert Ultra) has improved sensitivity for TB detection compared to the original Xpert MTB/RIF. Which specific technical modification in Ultra accounts for its higher sensitivity in paucibacillary specimens such as CSF?
- A Use of nested PCR to amplify rpoB from a single AFB per specimen
- B Isothermal loop amplification (LAMP) replacing PCR for lower inhibitor sensitivity
- C Increased reaction volume chambers (two reaction chambers instead of one) and addition of IS6110 and IS1081 targets alongside rpoB ✓
- D Addition of LJ medium integration for simultaneous culture confirmation
Explanation
Xpert MTB/RIF Ultra incorporates two key enhancements over the original Xpert: (1) larger sample processing volume with dual hemi-nested PCR reaction chambers increasing the number of target copies amplified, and (2) addition of two multicopy insertion sequence targets — IS6110 (present in 1–25 copies per MTB genome) and IS1081 (6 copies) — alongside the rpoB region. These multicopy targets dramatically improve sensitivity for paucibacillary specimens (CSF, pleural fluid, lymph node). The overall LOD improves from ~131 CFU/mL (original) to ~15.6 CFU/mL (Ultra). The tradeoff is slightly lower specificity due to detection of non-viable bacillary DNA in previously treated TB.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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Written and medically reviewed by the StethoPrep medical team.