TB is an often deadly, serious lung infection. Some forms of TB are resistant to antibiotic drugs, making it important for physicians to identify the correct TB strain and prescribe an appropriate treatment. Doctors often make a preliminary diagnosis of TB by collecting a sample of lung secretions and examining the sample under the microscope. To make sure of the diagnosis and to identify the TB strain that is causing the infection, the sample is sent to a laboratory for a follow-up bacteria culture test.
It may take as long as 6 weeks for the culture test to show a positive result, and it can take up to 6 more weeks to determine whether a child has a strain of TB that is resistant to the usual anti-TB drugs available. Because children have lower levels of infectious bacteria then do adults and have difficulty giving respiratory samples for examination, it is more difficult to detect the bacteria under a microscope and to grow the culture when the patient is a child. For these reasons, accurately diagnosing TB in children is difficult.
Researchers Supported by the Maternal and Pediatric Infectious Disease Branch recently evaluated a new type of TB test for use in children. This new test, known as Xpert MTB/RIF, was much more accurate than the preliminary microscope test, although less accurate than a bacterial culture test. However, the test also identified children with clinical symptoms of TB who responded well to TB treatment, but who had a negative bacteria culture test. This situation might occur when the bacteria are present at levels too low to be detected by culture or when the sample taken was not adequate. Importantly, the Xpert MTB/RIF could be completed rapidly: within 24 hours on average.
This test could be especially useful for children in resource-limited countries where culture tests can be difficult to obtain. Moreover, the test was very accurate in ruling out TB for children who did not have it. Finally, although there were no children with drug-resistant TB identified in this study, Xpert MTB/RIF can detect strains of TB that are resistant to the key anti-TB drug, rifampin. Resistance to rifampin occurs in both multidrug resistant (MDR) and extensively drug resistant (XDR) TB. The treatments for MDR and XDR TB are very different than the treatment for drug-sensitive TB.
Physicians can use this test to quickly rule out most cases of drug sensitive and drug resistant TB, preventing inappropriate treatment (PMID: 22752518).