A 35-year-old patient with pulmonary tuberculosis fails to respond to Category I DOTS regimen after 5 months. Sputum culture on LJ medium grows M. tuberculosis. Line probe assay (LPA — MTBDRplus) shows absence of wild-type band and presence of mutant band for rpoB codon 531 (Ser531Leu). No inhA or katG mutation detected. What is the drug susceptibility profile of this isolate?
- A Rifampicin resistant, isoniazid sensitive (likely MDR-TB confirmed on DST)
- B Rifampicin and isoniazid resistant (MDR-TB confirmed)
- C Sensitive to both rifampicin and isoniazid — repeat sputum needed
- D Rifampicin resistant, isoniazid status undetermined by this assay ✓
Explanation
MTBDRplus LPA detects rpoB mutations for RIF resistance and inhA/katG mutations for INH resistance. An rpoB codon 531 mutation confirms rifampicin resistance. However, MTBDRplus has ~80% sensitivity for INH resistance—high-level resistance is detected by katG315 mutations, and low-level by inhA promoter mutations, but not all INH-resistant strains are covered. Since no inhA or katG mutation is detected, INH resistance is neither confirmed nor excluded; phenotypic DST is still required for isoniazid. By WHO definition, RIF resistance alone triggers MDR-TB treatment pathway pending full DST.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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