Why is Xpert MTB/RIF not recommended for assessing treatment response in a patient already on antitubercular therapy?

• A Its sensitivity falls sharply once therapy has begun
• B MTB DNA persists after bacillary death, so positivity does not distinguish live from killed organisms ✓
• C Prior antibiotic exposure permanently abolishes the rpoB signal
• D Its internal control fails in post-treatment specimens

Explanation

NAAT detects DNA regardless of viability, and MTB DNA remains amplifiable for weeks after effective killing begins. A positive Xpert during follow up therefore cannot separate treatment failure from residual dead bacilli. Culture, which grows only viable organisms, remains the reference standard for monitoring, with smear as a practical adjunct. Sensitivity does not fall because of prior therapy, antibiotics do not erase the rpoB target, and the internal control functions normally.

Reference: Mandell, Douglas and Bennett's Principles and Practice of Infectious Diseases, 9th ed.

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Written and medically reviewed by the StethoPrep medical team.