A patient on a standard anti-TB regimen for 3 months remains sputum smear-positive. Drug susceptibility testing via Line Probe Assay (LPA) shows rpoB gene mutation (D516V) and absence of wild-type band for rpoB. What do these findings indicate?

• A Isoniazid resistance only
• B Rifampicin resistance — patient has at minimum RR-TB ✓
• C Susceptibility to all first-line drugs
• D Pyrazinamide resistance via pncA mutation

Explanation

The rpoB gene encodes the beta-subunit of RNA polymerase, the target for rifampicin. A D516V mutation in rpoB causes high-level rifampicin resistance. In LPA (GenoType MTBDRplus), absence of wild-type band plus presence of a mutation band for rpoB confirms rifampicin resistance, classifying the patient as at minimum RR-TB (rifampicin-resistant TB). Since rifampicin resistance is used as a surrogate for MDR-TB in clinical practice (per WHO), these patients are managed with MDR-TB regimens pending full DST. Isoniazid resistance is detected by katG and inhA mutations on the same LPA.

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

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Written and medically reviewed by the StethoPrep medical team.