Rapid Test Allows For Earlier Diagnosis of TB in Children
Preliminary diagnosis of TB is often made by collecting a sample of lung secretions and examining the sample under a microscope to see if it contains the bacteria that cause TB. A sample is also sent to a laboratory so the bacteria can be cultured and identified. It may take as long as six weeks for the culture test to show a positive result. Because, children have lower levels of infectious bacteria than do adults, it is more difficult to detect the TB bacteria under a microscope and to grow it in a culture. For this reason, accurately diagnosing TB in children has been difficult.
According to the results of a study in South Africa published in The Lancet Global Health (2013; 1:e97-e104) and supported by the NIH, a new test for diagnosing TB in children detects roughly two-thirds of cases identified by the current culture test, but in a fraction of the time. The test, known as Xpert MTB/RIF, also detected five times the number of cases identified by examining specimens under the microscope, a preliminary method for diagnosis that is often performed as an initial test, but which must be verified by the culture test.
Xpert MTB/RIF results from respiratory secretions were ready in 24 hours, on average, compared with an average of more than two weeks for the culture test used in the study. Previous studies have shown that Xpert MTB/RIF is effective for diagnosing TB in adults and in children with pronounced symptoms of TB who have been admitted to a hospital. Diagnosing TB in children is more difficult than diagnosing it in adults, because children tend to have much lower levels of the TB bacteria than do adults. The results of the current study indicated that the ease and speed of diagnosis would be useful for children seen in clinics in resource-limited countries, which often lack the resources for traditional testing that are available in hospitals. The test also was able to identify children with drug resistant TB. In addition, the authors found that Xpert can readily determine when treatment for tuberculosis is not appropriate. Among children who did not in fact have TB, the results of the Xpert test came back negative for TB with 99% accuracy. The Xpert MTB/RIF test also detects TB strains that are resistant to the drug rifampicin, allowing physicians to more accurately prescribe an appropriate treatment. This is particularly important in areas where drug-resistant TB is common, such as South Africa.
The study collected almost 1,500 samples from nearly 400 children who went to a primary care clinic with symptoms of TB. Collecting the samples — secretions from the lungs, the nasal passages or both — requires special equipment and trained clinical staff. The authors compared the results from the Xpert MTB/RIF test, examination of samples under a microscope, and from growing the tuberculosis bacteria in laboratory cultures. Bacterial culture is the most accurate method for diagnosing TB. Of the 30 TB cases detected by culture, 19 (63%) were positive by the Xpert MTB/RIF test on lung or nasal samples, while examining the samples under the microscope turned up only four cases (13%). Adding a second test (of a second lung or nasal passage sample) improved the detection rate for both culture and Xpert MTB/RIF
In some cases, the authors started TB treatment for children they suspected had TB, based on their symptoms. Xpert MTB/RIF identified seven children who had clinical symptoms of TB and responded well to treatment, but whose TB had not been detected by the TB culture test. This might occur when a child is sick with TB, but the bacteria are at especially low levels, or because a sample did not contain enough of the bacteria present in the child’s body to appear when cultured. The total number of cases detected by culture (30 cases) and by XpertMTB/RIF (26 cases) was similar.