A sputum sample from a suspected TB patient is processed using the NALC-NaOH decontamination method. GeneXpert MTB/RIF (Xpert) is performed and returns: MTB DETECTED; Rifampicin RESISTANCE DETECTED. The next step for comprehensive drug susceptibility testing (DST) is:

• A Directly start XDR-TB regimen with bedaquiline + linezolid + pretomanid, as Xpert RIF resistance is sufficient
• B Repeat Xpert on a second sputum sample before performing any DST, as a single Xpert RIF-resistance result is unreliable
• C Send sample for phenotypic MGIT 960 liquid culture only; molecular DST is not WHO-approved for clinical management decisions
• D Perform Line Probe Assay (LPA) — GenoType MTBDRplus — to detect additional isoniazid resistance (inhA and katG) and confirm RIF mutations; reflex to second-line LPA (MTBDRsl) for FQ and injectable resistance ✓

Explanation

WHO guidelines recommend that a confirmed RIF-resistant result on Xpert should trigger reflex molecular DST using GenoType MTBDRplus (first-line LPA), which simultaneously identifies mutations in rpoB (RIF), katG (INH high-level resistance) and inhA promoter (INH low-level resistance), enabling early classification as MDR-TB. Further, GenoType MTBDRsl (second-line LPA) detects resistance to fluoroquinolones (gyrA/B) and group A injectables (rrs, eis), guiding pre-XDR or XDR-TB identification. Starting bedaquiline/linezolid/pretomanid without full DST would be premature. Repeating Xpert is not required if quality criteria are met. MGIT culture is complementary but should not delay molecular DST.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.