What Is the GeneXpert MTB/RIF System?

Tuberculosis (TB) is one of the leading infectious diseases affecting children worldwide. Diagnosing TB in children has always been a challenge because children with TB often have very few bacteria in their body, making traditional tests unreliable. The GeneXpert MTB/RIF System, also called the Xpert MTB/RIF assay, is a molecular diagnostic device that changed this situation.

Developed by Cepheid, the GeneXpert system uses a technology called real-time polymerase chain reaction (PCR) to detect the DNA of the tuberculosis bacterium (Mycobacterium tuberculosis) directly from a patient sample. At the same time, it checks whether the bacteria is resistant to rifampicin, which is one of the most important TB medicines. All of this happens inside a single-use plastic cartridge, in approximately 90 minutes, with very little manual work involved.

In 2010, the World Health Organization (WHO) recommended this test for TB diagnosis globally. It has since become a cornerstone of TB control programs in more than 130 countries, and its newer version, the GeneXpert MTB/RIF Ultra, is now especially recommended for children, where detecting small amounts of TB DNA is critical.

Purpose and Where the Device Is Used

Main Purpose

The GeneXpert MTB/RIF system serves two major functions at the same time:

• Detection of TB: It finds the genetic material (DNA) of Mycobacterium tuberculosis in a patient sample, confirming whether TB infection is present.
• Drug resistance detection: It identifies mutations in the rpoB gene of the TB bacterium, which indicate resistance to rifampicin. Rifampicin resistance is a strong marker for multi-drug resistant TB (MDR-TB), because in most cases, resistance to rifampicin goes along with resistance to isoniazid, another core TB drug.

Where It Is Installed and Used

Specimens Accepted

In adults, sputum is the standard specimen. In children, who often cannot produce sputum on their own, other specimens are used:

• Induced sputum (sputum triggered by inhaling a saline mist)
• Gastric aspirate (stomach fluid collected in the early morning)
• Bronchoalveolar lavage (BAL) from the lungs
• Cerebrospinal fluid (CSF) - for TB meningitis
• Lymph node aspirates or tissue biopsies
• Pleural fluid
• Stool samples (Ultra version only, especially useful in young children)

Types and Models Available

The GeneXpert platform is a modular system. Different instrument sizes are available depending on how many tests need to be done at a time. The cartridges used are the same across most platforms.

Cartridge (Assay) Types

Instrument Models (by number of modules)

Each module in the instrument works independently. This means a 4-module machine can run 4 different patient tests at the same time, each potentially starting at a different time, without one affecting another.

How the GeneXpert MTB/RIF System Works

Understanding the science behind this device helps in appreciating why it is so reliable.

The Technology: PCR in a Cartridge

The Xpert MTB/RIF test uses a method called hemi-nested real-time PCR. PCR (polymerase chain reaction) is a technique that makes millions of copies of a specific piece of DNA so it can be detected easily. Here is what happens inside each cartridge:

1. Sample enters the cartridge The patient's sputum (or other specimen) is mixed with a special sample reagent (containing sodium hydroxide and isopropanol) and transferred into the multi-chambered plastic cartridge. This reagent also kills the TB bacteria, making the sample safer to handle.
2. Bacteria are captured and broken open Inside the cartridge, the TB bacteria are filtered out and concentrated. They are then broken open using a process called sonication (high-frequency sound waves), releasing their DNA.
3. DNA amplification (PCR) The machine amplifies (copies) a specific part of the TB bacterium's DNA - the rpoB gene region - millions of times so it becomes detectable.
4. Detection using molecular beacons Special fluorescent probes called molecular beacons bind to the copied DNA. Five overlapping probes (labeled A, B, C, D, and E) scan the rpoB gene for any mutations that signal rifampicin resistance.
5. Automated result generation The machine's software reads the fluorescent signals and generates the final result automatically. The entire process takes approximately 90 minutes. No manual steps are needed after loading the cartridge.

Because all steps happen inside the sealed cartridge, there is almost no risk of sample contamination or laboratory exposure to live TB bacteria.

Step-by-Step User Guide

The following outlines the standard procedure for running the GeneXpert MTB/RIF test. Actual steps may vary slightly depending on the specific instrument model and institutional protocol. This guide follows general manufacturer and WHO-aligned recommendations.

Equipment and Materials Needed

• GeneXpert instrument (connected to a power supply and computer with GeneXpert software)
• Xpert MTB/RIF or Ultra cartridge (single-use, stored at 2-28 degrees C)
• Sample reagent (SR) - provided with the cartridge kit
• Patient specimen (sputum, gastric aspirate, BAL, etc.)
• Transfer pipette or dropper
• Gloves, lab coat, and appropriate PPE
• Biohazard waste bag

Before Starting: Check These Points

Step-by-Step Procedure

1. Prepare the specimen For raw sputum: add sample reagent (SR) to the specimen in a 2:1 ratio (2 parts SR to 1 part sputum). Close the tube and mix by inverting 10 times. Let it sit at room temperature for 15 minutes, then mix again 10 times before use. For induced sputum or other specimen types, follow the specific preparation protocol for that specimen.
2. Open the cartridge Peel back the cartridge label or open the cartridge lid. Do not touch the cartridge's internal components.
3. Transfer the prepared sample Using the transfer pipette, fill the cartridge with the treated specimen to the "fill line" marked on the cartridge. Close the cartridge lid firmly until it clicks.
4. Register the test in the software On the connected computer, open the GeneXpert software, create a new test entry, and enter the patient sample ID and test type. Select the module (numbered slot) where the cartridge will be placed.
5. Load the cartridge into the instrument Open the GeneXpert instrument's module door. Insert the loaded cartridge into the module, label side facing outward. Close the module door. Start the test must begin within 4 hours of adding the sample reagent to the specimen.
6. Start the test In the software, confirm the module assignment and click "Start Test." The instrument will run all remaining steps automatically. No further manual steps are needed.
7. Wait for results The test runs for approximately 90 minutes. The software will notify when the result is ready. Do not open the module door during the run.
8. Read and record the result The result will appear on the computer screen. Print or record the result as per institutional protocol. Remove the used cartridge with gloves and dispose of it in a biohazard waste container. Used cartridges should be treated as potentially infectious waste.

Understanding the Results

The GeneXpert system produces clearly labelled results. Here is what each result means:

Precautions and Safety Points

For Laboratory and Healthcare Staff

• Always wear gloves when handling patient specimens. Add a lab coat and, where appropriate, eye protection when preparing samples.
• The sample reagent (SR) contains sodium hydroxide and isopropanol, both of which are corrosive. Avoid skin and eye contact. In case of spills, rinse with water immediately.
• The sample reagent is tuberculocidal - it kills the TB bacteria. Once the specimen has been mixed with SR and transferred into the cartridge, the risk of TB infection from the sample is greatly reduced. However, standard biosafety precautions should always be maintained.
• Never open a cartridge once the test has started. The cartridge is a sealed unit; opening it can release aerosols.
• Dispose of used cartridges as biohazardous waste in accordance with local healthcare waste disposal regulations.
• Do not attempt to clean the interior of the instrument with liquids.

For Specimen Collection in Children

• Gastric aspirate must be collected in the early morning, before the child eats or drinks, for best results.
• Induced sputum collection requires a trained team and appropriate equipment; it should not be attempted without the right setup.
• Specimens should be stored at 2-8 degrees C and processed as soon as possible. Raw sputum can be stored for up to 8 hours at room temperature or refrigerated for up to 7 days before testing.

Device and Cartridge Handling

• Cartridges must be stored at 2-28 degrees C. Do not freeze cartridges.
• The instrument must operate at room temperature between 15 degrees C and 30 degrees C. Operating outside this range increases error rates.
• The instrument requires a stable, uninterrupted electricity supply. Power cuts during a test will cause errors and wasted cartridges.
• The instrument modules must be professionally calibrated once a year. Uncalibrated instruments can give inaccurate results.
• Cartridges must not be reused. Each cartridge is single-use only.
• Do not use cartridges past their expiry date.

How to Keep the Device and Cartridges Safe

Cartridge Storage

• Store cartridges at 2-28 degrees C (refrigerator temperature is fine). The manufacturer states cartridges can tolerate up to 45 degrees C for short periods (under 6 weeks at 75% humidity), but routine storage should stay within the recommended range.
• Store cartridges in their original packaging until ready to use.
• Do not expose cartridges to direct sunlight or moisture.
• Check expiry dates before every use and rotate stock using a first-in, first-out system to avoid waste.
• Cartridges are bulky when boxed. Ensure there is adequate storage space at the facility.

Instrument Care

• Keep the instrument in a clean, dust-free area. Dust buildup inside modules can affect optical readings.
• Maintain room temperature between 15-30 degrees C at all times. Air conditioning may be necessary in hot climates.
• Use a voltage stabilizer or uninterruptible power supply (UPS) to protect against power fluctuations and outages.
• Clean the exterior of the instrument with a damp cloth. Do not use harsh chemicals or sprays near or inside the instrument.
• Schedule annual professional calibration of all modules. Keep a log of calibration dates and any error codes.
• Report any persistent error codes or module failures to a certified Cepheid service technician. Do not attempt to repair internal components.

Software and Data

• Regularly back up the GeneXpert software database to prevent loss of patient result records.
• Ensure the computer connected to the instrument has adequate security (password protection, updated antivirus software).
• Keep the GeneXpert software updated as per manufacturer recommendations.

Additional Information Relevant to Clinical Use

GeneXpert in HIV-Positive Children

Children with HIV are at high risk of TB and may have atypical disease presentations. The GeneXpert system is particularly valuable in this group because:

• HIV-positive children often have a higher bacterial load, making the test more sensitive in some scenarios.
• They are also more likely to have extrapulmonary TB, where GeneXpert (especially Ultra) can test non-sputum specimens.
• WHO specifically recommends treating "trace" results as positive in HIV-positive individuals, the same as in children.

Role in MDR-TB Identification

Multidrug-resistant TB (MDR-TB) is a serious and growing concern in many parts of the world. The GeneXpert system's ability to detect rifampicin resistance within 90 minutes is a critical tool in quickly identifying children who need specialized MDR-TB management, rather than waiting weeks for culture results. Early identification directly improves treatment outcomes and limits further spread.

GeneXpert as Part of TB Care

GeneXpert is a starting point, not the entire diagnostic process. After a positive result, patients require a full clinical assessment, chest X-ray, treatment planning, and follow-up. A negative result in a strongly suspected case requires further workup. The system is best seen as one important tool within a complete TB care pathway.