Cartridge Based Nucleic Acid Amplification Test (CB-NAAT) is a fast and highly accurate test for diagnosis of tuberculosis (TB). The CB-NAAT detects tuberculosis (TB) Bacteria or Mycobacterium tuberculosis and simultaneously detects resistance to Rifampicin, which is the key drug in the treatment of tuberculosis (TB). CB-NAAT can be performed on 2 platforms, GeneXpert/ Xpert MTB/RIF assay or a TruNat platform. CB-NAAT has been endorsed by the WHO and the Government of India as the initial diagnostic test for the diagnosis of Tuberculosis (TB). In this article we shall discuss the Genexpert test.
How is the GeneXpert test performed?
The sample which needs to be tested for GeneXpert test is placed in a small tube. From the tube, the sample is fed into the cartridge of the machine. Further steps are automated and the system detects presence of tuberculosis (TB) bacteria and simultaneously resistance to rifampicin.
What are the advantages of a Gene Xpert test?
The GeneXpert test is very accurate in the diagnosis of tuberculosis (TB) compared to the smear examination which was performed earlier. Genexpert test is able to detect tuberculosis (TB) bacilli in any sample, even when the concentration is as low as 50 to 100 bacteria per mL. The traditional smears used for diagnosing tuberculosis (TB) were able to detect when the bacteria load was approximately 10,000 bacteria per mL.
The GeneXpert test is simple to perform and does not involve many complex steps. This allows the GeneXpert test to be performed by minimally trained technicians even at peripheral district centres. Since the test is run inside a sealed cartridge, the chances of contamination are minimal.
Gene Xpert test gives quick results. The time taken from feeding the sample into the cartridge to the final result takes approximately 80-90 minutes. This helps in early diagnosis and treatment of the patient on the same day when the test is done. It also needs to be remembered that the traditional culture test used for diagnosis of tuberculosis (TB) took approximately 3 to 4 weeks.
What sample is used for GeneXpert?
The sample which is used for the genexpert test depends on the site of involvement in tuberculosis (TB). Sputum is the most common sample used for GeneXpert Test if the child is suspected to have tuberculosis (TB) of the lungs. In children who are unable to bring out sputum, either a gastric lavage or an induced sputum sample is collected and subjected to a GeneXpert test. In case we are unable to make a diagnosis from a gastric lavage or an induced sputum, a bronchoscopy and bronchoalveolar lavage (BAL) is performed. The bronchoalveolar lavage (BAL) sample is then subjected to the GeneXpert test.
In addition to the above samples we can also subject pleural fluid (fluid between the layers of the pleura outside the lungs), peritoneal fluid (fluid in the abdomen), cerebrospinal fluid (fluid around the brain and spinal cord) and pus samples to GeneXpert test.
How accurate is GeneXpert for TB?
The GeneXpert test is accurate in diagnosing pulmonary tuberculosis (TB) and its greatest benefit is demonstrated in smear-negative tuberculosis (TB) patients. The sensitivity of the current GeneXpert test is almost equivalent to a culture test. The sensitivity and the specificity of GeneXpert test in children is 70% and 90% respectively. Sensitivity and the specificity of a genexpert test in samples other terms sputum is lower but still much better compared to the previous smear test which were done.
