Why is the Xpert MTB/RIF assay unsuitable for monitoring bacteriological response during anti-tuberculosis therapy, even though it is highly sensitive at diagnosis?

• A Rifampicin in the patient's serum interferes chemically with the probe hybridisation step
• B DNA from non-viable bacilli persists in sputum after effective treatment, giving persistently positive results ✓
• C The cartridge becomes saturated once the patient has received more than two months of therapy
• D Mutations selected during therapy make the assay systematically false negative

Explanation

Xpert amplifies nucleic acid, and mycobacterial DNA remains detectable in sputum long after the bacilli have been killed by effective chemotherapy. A patient converting to culture negativity can therefore remain Xpert positive for weeks to months, so the assay cannot distinguish live from dead organisms and is not recommended for treatment monitoring. Smear microscopy and solid or liquid culture, which depend on viability, remain the tools for follow-up. Probe chemistry, cartridge capacity and systematic false negatives are all incorrect explanations.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

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Written and medically reviewed by the StethoPrep medical team.