According to the World Health Organization (WHO), about one-quarter of the world’s population has a tuberculosis (TB) infection. These numbers can be even higher in TB endemic areas like Africa. 

One of the primary methods used to detect tuberculosis and its resistance to one of the drugs often used to treat it—rifampin—is with a revolutionary test called Xpert MTB/RIF.  

The Centers for Disease Control and Prevention note that the test “simultaneously detects Mycobacterium tuberculosis complex (MTBC) and resistance to rifampin (RIF) in less than two hours. In comparison, standard cultures can take two to six weeks for MTBC to grow, and conventional drug-resistance tests can add three more weeks.”

“The use of this assay in TB endemic settings such as Kenya has been a game-changer for TB diagnosis,” added Kimberly McCarthy, a tuberculosis researcher based in Kenya and a USF College of Public Health DrPH candidate studying public health and clinical laboratory science and practice. “Microscopy is the primary method for TB diagnosis in most resource-constrained, high TB-burden settings. While cost-effective, microscopy has very low sensitivity, especially in children who generally present with paucibacillary [containing few bacilli] TB disease.”

But the assay isn’t foolproof, and, in fact, can give false-positive rifampin results, especially in settings like Kenya where resistance to rifampin is low among individuals who haven’t been exposed to antituberculosis therapy.

To determine the extent of the problem and to develop a better diagnostic method for TB among children, McCarthy and her colleagues looked at 300 symptomatic children, 11 percent of whom were positive for TB. 

“Of these, 3 of 27, or 11 percent, were resistant to rifampin according to the Xpert MTB/RIF, however this was not confirmed based on additional molecular testing, patient contact history or response to a rifampin-containing drug regimen,” she said. “In settings with a low prevalence of drug-resistant TB, including rifampin resistance, the Xpert MTB/RIF may yield false-positive rifampin resistance results. We advise following WHO guidelines, which recommend confirmatory testing.”

McCarthy and her colleagues published their findings, titled “The Need for Caution When Interpreting Xpert MTB/RIF Results for Rifampin Resistance Among Children” in November in the International Journal of Tuberculosis and Lung Disease.

“This research demonstrates the utility of a molecular assay such as the Xpert MTB/RIF as a tool for diagnosing TB in high TB-burden settings and describes potential assay limitations,” McCarthy said. “We respectfully share these findings with the global community as a reminder to perform further TB testing when rifampin resistance is detected in settings where resistance to rifampin among individuals naive to TB therapy is low.”

Story by Donna Campisano, USF College of Public Health