New Test for Rapid Diagnosis of Tuberculosis in Children

South Africa: A new test for diagnosing tuberculosis (TB) in children detects roughly two-thirds of cases identified by the current culture test, but in a fraction of the time, according to the results of a study in South Africa supported by the National Institutes of Health. This allows two thirds of the children to start treatment in 24 hours instead of waiting six weeks for lab results.

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, the researchers found. 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
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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 researchers 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 percent accuracy.

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 bacteria under a microscope and to grow it in a culture. For this reason, accurately diagnosing TB in children has been difficult.

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, said Carol Worrell, M.D. This is particularly important in areas where drug-resistant TB is common, such as South Africa.

The World Health Organization estimated that in 2011 there were 500,000 TB cases and 64,000 deaths among those younger than 15 years.