Xpert TB test may be effective in low-burden settings
An automated nucleic acid amplification test widely used for early detection of tuberculosis in countries where the disease is more prevalent also worked well in the United States, suggesting the technology could be effective in low-burden settings, according to a study published in Clinical Infectious Diseases.
“These data support the use of [Xpert MTB/RIF test (Cepheid)] as a diagnostic tool in U.S. patients undergoing initial evaluation for TB,” researchers wrote.
Their study looked at 992 adult patients with suspected pulmonary TB from 21 sites in the U.S. and in Rio de Janeiro and Johannesburg — two locations with a high burden of the disease — between May 2012 and November 2013.
The researchers compared Xpert testing to two sputa evaluated with acid-fast bacilli (AFB) smears and mycobacterial cultures using liquid and solid culture media.
Thirteen percent of participants had a history of prior TB, and 19% had initiated TB treatment less than 48 hours before sputum was collected. Twenty-two percent of the participants had culture-confirmed TB, including 14% of U.S. participants and 36% of the participants from the other two sites.
The researchers found that a second Xpert test in the U.S. increased the overall sensitivity from 85.2% to 91.1% — including 100% in participants with positive smears — while specificity declined slightly from 99.2% to 98.9%. The negative predictive value increased from 97.6% to 100% with two Xpert tests, suggesting a role for the test in determining if patients should be removed from airborne infection isolation, according to the researchers.
The results were similar to those from Rio de Janeiro and Johannesburg, and comparable with other studies in places with a higher prevalence of TB, the researchers said.
The researchers were limited in their ability to study rifampin resistance because just seven participants demonstrated resistance.
“However, Xpert demonstrated a high NPV of 98.9% for exclusion of rifampin resistance,” the researchers wrote. “Given the importance of prompt, accurate identification of rifampin resistance, rapid molecular testing is recommended to confirm rifampin resistance detected by Xpert, and culture-based drug susceptibility testing is recommended in all U.S. TB cases.”
In a related editorial, Andrew R. DiNardo, MD, from the Global TB Program at Texas Children’s Hospital, and colleagues said the study provided “compelling evidence” for using Xpert in countries where TB is less prevalent, and noted that it is easier to administer and produces faster results compared with other nucleic acid amplification tests.
“Now, the compelling evidence of Xpert’s improved sensitivity and negative predictive value compared to microscopy must guide clinical practice in the United States and other countries with low-burden of tuberculosis,” DiNardo and colleagues wrote.
Disclosure: Please see the full study and editorial for a list of all authors’ relevant financial disclosures.