Xpert MTB/RIF Ultra: Introduction, Principle, Procedure, Result-Interpretation, Uses, and Keynotes
GeneXpert Dx System software interface displaying a completed diagnostic result for an Xpert MTB-RIF Ultra assay
Introduction
Table of Contents
Xpert MTB/RIF Ultra is an automated diagnostic test designed for the rapid and simultaneous detection of Mycobacterium tuberculosis (MTB) and resistance to rifampin (RIF). It is a second-generation assay developed by Cepheid to provide significantly higher sensitivity than the original Xpert MTB/RIF, particularly for patients with low bacterial loads, such as children and people living with HIV.
Principle
The test utilizes nested real-time polymerase chain reaction (PCR) and melt peak detection technology.
Target DNA: It amplifies therpoB gene (to detect RIF resistance) and two multi-copy insertion elements, IS6110 and IS1081 (to enhance MTB detection sensitivity).
Automation: All steps—from sample purification and DNA extraction to amplification and detection—occur within a single-use, self-contained cartridge to minimize cross-contamination.
Controls: Each cartridge includes a Sample Processing Control (SPC) to monitor for PCR inhibitors and a Probe Check Control (PCC) to verify reagent integrity and tube filling.
Procedure
The process is streamlined into three main phases, typically yielding results in approximately 80 minutes.
Fig. GeneXpert IV 4-module system
Sample Preparation:
Sputum is mixed with a provided Sample Reagent (SR) (usually at a 2:1 ratio for unprocessed sputum).
The mixture is shaken vigorously and incubated at room temperature for 15 minutes (with a second shake after 10 minutes) until fully liquefied.
Cartridge Loading:
Approximately 2 mL of the liquefied sample is transferred into the GeneXpert cartridge using a sterile pipette.
Analysis:
The cartridge is inserted into the GeneXpert Instrument System, where the automated PCR process begins.
Fig. The image shows a single GeneXpert® Xpert® MTB/RIF Ultra diagnostic cartridge. It features a blue flip-top lid, a clear plastic body, and a white label with a QR code. The label indicates a LOT number of 63507 and an expiry date of 2026-12-27.
Result Interpretation
Results are categorized based on the detection of MTB DNA and mutations in the rpoB gene.
Result
Interpretation
MTB Detected (High, Medium, Low, Very Low)
MTB is present; RIF resistance is reported as “Detected,” “Not Detected,” or “Indeterminate”.
MTB Trace Detected
Lowest limit of detection (paucibacillary sample). RIF resistance cannot be determined (reported as indeterminate).
MTB Not Detected
No MTB DNA was identified within the sample’s detection limit.
Invalid / Error / No Result
The test failed due to factors like inhibitors or technical errors; a repeat test is required.
Fig. GeneXpert Dx System software interface displaying a completed diagnostic result for an Xpert MTB-RIF Ultra assay
Uses
Initial Diagnosis: Recommended by the WHO as the primary diagnostic test for all individuals with suspected pulmonary TB.
Drug Resistance Screening: Rapidly identifies patients at risk of multidrug-resistant TB (MDR-TB).
Paucibacillary TB: Highly effective for diagnosing smear-negative cases, extrapulmonary TB (e.g., CSF for meningitis), and TB in children.
Keynotes
Limit of Detection (LOD): Ultra has a much lower LOD (approximately 16 CFU/mL) than the original version (114 CFU/mL), making it nearly as sensitive as liquid culture.
The “Trace” Result: While highly sensitive, a “trace” result in adults with a recent history of TB (within 2 years) may reflect non-viable (dead) bacilli rather than active infection.
Not for Monitoring: The test cannot be used to monitor treatment progress because it detects both live and dead DNA.
Confirmatory Testing: RIF resistance detected in low-prevalence areas should ideally be confirmed by another method or a second specimen.
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