How GeneXpert Test Can Detect Tuberculosis in Minutes

How is the GeneXpert Test Done? 

The GeneXpert Test is a highly advanced molecular diagnostic test that is widely used for the detection of various infectious diseases. It is a breakthrough technology that combines automated sample preparation, nucleic acid amplification, and real-time polymerase chain reaction (PCR) analysis, all in a single cartridge.  

To understand the GeneXpert test you need to know how it is done. This rapid diagnostic test rapid diagnostic test has a very high sensitivity and can be used to test lung diseases such as COVID and tuberculosis (TB).  

Given below is the step-by-step breakdown of a GeneXpert test: 

• The first step of the GeneXpert test procedure is the collection of a biological sample. A specific test is conducted based on the sample taken, i.e., blood, urine, sputum or cerebrospinal fluid.  

• The sample is then processed to extract the DNA or RNA from the cells. At this step, the cells are broken open, and the genetic material is separated from the other debris of the cell before it can be purified for analysis.  

• This purified sample is mixed with the appropriate reagents and loaded into a disposable test cartridge that contains all the components needed for molecular analysis. This cartridge is specifically designed for the GeneXpert system sample.  

• The test cartridge is loaded into the GeneXpert system, which then takes over and automates the rest of the steps, performing reverse transcriptase, amplification, and detection of the target genetic materials with the help of real-time PCR (polymerase chain reaction) technology. 

• By continuously monitoring the PCR reaction, this GeneXpert system can determine whether or not the target genetic material is present in the sample and also quantifies the amount of genetic material current, giving doctors a real-time analysis of the results.  

• Once the process is complete, the GeneXpert system comes up with a test report indicating the results, including the detected microorganism, the genetic characteristics it displays, and any other information like its drug resistance profile.  

How Much Time Does GeneXpert Take? 

Traditional TB tests that required growth of a standard culture, usually took 3 to 5 weeks, which does not include the additional three weeks needed to identify drug resistance. The GeneXpert TB test takes a fraction of the time as it is swift and fully automated. This new test can detect acid-fast bacilli and drug resistance to a drug called rifampicin in just a couple of hours.  

Which is Better? TB PCR Test or GeneXpert?  

GeneXpert is the newest technology with many benefits over traditional TB PCR tests. Here are a few significant reasons why the GeneXpert test is better than conventional TB PCR:  

• Speed and efficiency: GeneXpert gives you quick automated reports within a few hours. This is significantly faster than traditional TB PCR tests, which would take several hours and even days to give you the same result.  

• Ease of use: The GeneXpert test is designed with the user in mind. Its fully integrated automated system needs lower technical expertise than traditional tests and also reduces the chance of human errors during the testing process. Standard TB PCR tests involved multiple manual steps that required more technical efficiency and could be at risk of manual errors.  

• Detection of drug resistance: The GeneXpert test simultaneously detects TB and determines drug resistance, especially for rifampicin, which can indicate the presence of multi-drug resistant tuberculosis. This can be crucial information that helps direct treatment decisions and guide the patients towards the right disease management path. On the contrary, traditional TB PCR tests required extra time and added steps to determine drug resistance.  

• Point of Care testing: GeneXpert is portable and can be used in a decentralised setting, this test can be used in rural areas and reduces the turnaround time previously associated with TB PCR testing.  

What Happens if GeneXpert is Negative?  

• If your GeneXpert TB results come back negative and you are not on TB treatment, it indicates that you do not suffer from tuberculosis and do notice, do not need to take any further actions.  

• If you get a negative GeneXpert test result and are currently on TB treatment for an active disease, it could mean that you previously had tuberculosis but have had successful treatment and may be cured. You may need to approach one of many TB clinic physicians to review your entire case and decide if you can stop your treatment of the illness or if you are still at risk of spreading this highly contagious disease and need to take further precautions.  

As no diagnostic test can replace the clinical judgement of your doctor, it is essential to discuss any diagnostic test with your physician and come up with a well-thought-out plan for future treatment or when you can stop TB treatment.  

What is the Limit of Detection for GeneXpert TB? 

The limit of detection (LOD) for GeneXpert TB depends on the target being detected, namely. 

• LOD for Mycobacterium tuberculosis: 131-250 colony-forming units per millilitre (CFU/ml) in the sputum GeneXpert test 

• LOD for GeneXpert MTB/RIF for rifampicin-resistant/multidrug-resistant TB: 5-10 CFU/ml, which means it can detect drug-resistant bacteria in the smallest samples.  

What Sample is Used for GeneXpert?  

A biological sample is needed for the GeneXpert test; blood, sputum, or tissue samples are commonly used to perform these tests on the GeneXpert system. The specific GeneXpert test sample type depends on the condition or disease that needs testing. In a tuberculosis trial, a sputum sample is usually required. For other respiratory diseases, you may need to include a respiratory secretion sample collected with the help of nasal or nasopharyngeal swabs.  

After the sample is collected, it is combined with a reagent containing the required components for the PCR reaction. This solution is then loaded into a disposable cartridge/test module that is inserted into the GeneXpert device. The entire test is carried out within this device as it monitors and amplifies the genetic material in real time to detect and quantify the target genetic material.  

The results for this test are often given within a few hours of inserting the sample into the GeneXpert device, depending on which test needs to be performed. The choice of the sample will rely mainly on the target condition or disease and the instructions provided with the GeneXpert device. Technicians need to follow the proper sample collection and handling procedures to get reliable and accurate results. 

How Accurate is GeneXpert? What is the Sensitivity and Specificity of the GeneXpert Test?  

The diagnostic efficiency of the GeneXpert device is assessed at 85 MTB culture-positive cases with an overall sensitivity, specificity, PPV and NPV at a 95% confidence interval of 98.6%. This result could vary based on whether the test is used singly or combined with other tests.  

If cough is used as the sole screening tool, the sensitivity, specificity, PPV and NPV would be found to be around 3.65%, 100%, 21.2%, and 100%, respectively. When fever and cough were considered the screening tool, these varied changed to 25.5%, 84.5%, 22.6%, and 86.8%, respectively. If any three symptoms of the screening were positive, it could lead to the sensitivity, specificity, PPV, and NPV to show 44%, 60%, 21%, and 80%, respectively.  

When conducting a GeneXpert test, technicians must consider various parameters in addition to these mentioned factors. 

Conclusion 

A GeneXpert TB test is one of the latest innovations in TB testing, it is quicker and more efficient than a traditional TB PCR test. While a GeneXpert test positive result indicates that you may have contracted TB, a negative result helps you and your doctor detect the absence of the microorganism causing the disease that can also be due to the treatment.  

Metropolis Labs is a leading provider of cutting-edge testing services, offering a wide range of diagnostic tests to cater to various medical needs. One of our notable offerings is the GeneXpert test, which utilises advanced technology to deliver accurate and timely results. Check out our entire list of diagnostic tests today.