CLN Article

IGRA Recommended Over TST for Suspected Latent TB Infection Work-Up

The Sample: March 2017

New guidance on diagnosing Mycobacterium tuberculosis (Mtb) infection generally favors interferon-gamma release assay (IGRA) over tuberculin skin test (TST) and recommends a risk-based approach to testing centered on the likelihood of a patient being infected with Mtb and the likelihood of any infection progressing to TB disease (Clin Infect Disease 2016; doi:10.1093/cid/ciw694). The guidelines, jointly devel-oped by the American Thoracic Society, Infectious Diseases Society of America, and Centers for Disease Control and Prevention, “are not intended to impose a standard care” but rather offer “a basis for rational decisions” in the diagnostic workup of patients with possible latent TB infection (LTBI) or TB, according to the authors.

The 18-member panel that developed the guideline recommended IGRA over TST in individuals at least 5 years old who meet four criteria, including having likely TB infection, a low- or intermediate-risk of disease progression, warranted LTBI testing, and either a history of bacille Calmette-Guérin (BCG) vaccination or judged unlikely to return for a TST reading. The authors emphasized, however, that TST would be an ac-ceptable alternative, especially when IGRA is not available, too costly, or too burdensome.

The authors concurred with other guidelines that suggest individuals at low risk of TB infection not be test-ed for Mtb infection. However, they acknowledged that laws or regulatory bodies might still require this test-ing. In such cases, the authors recommended IGRA over TST, but also suggested a second diagnostic test if the initial test is positive. Only when the two tests yield positive results would the patient be considered to have TB.

The guideline emphasizes that neither IGRA nor TST distinguish active TB infection from LTBI. Conse-quently, active TB infection needs to be excluded before a patient starts treatment for LTBI.

In the diagnostic workup of patients with suspected pulmonary TB, the panelists advised performing nucle-ic acid amplification testing on the patients’ initial respiratory specimen. They also recommended rapid mo-lecular drug susceptibility testing for rifampin in patients who have positive sputum or Hologic Amplified Mycobacterium Tuberculosis Direct Test results and who meet four criteria.