Wednesday, July 18, 2012

Which test should be used for TB screening?

- Kenny Lin, MD

The tuberculin skin test has a long and venerable history as the screening test of choice for latent tuberculosis infection in the United States. A 2009 AFP article called it "the most acceptable method" of tuberculosis screening. However, the tuberculin skin test has limitations: it requires 2 patient visits, and may be difficult to interpret in persons from other countries that have received bacille Calmette-Guerin (BGC) vaccinations. In contrast, interferon-gamma release assays (IGRAs) performed on blood samples require only a single patient visit, do not cross-react with BCG, and according to a recent systematic review, have comparable specificity to TSTs. The Centers for Disease Control and Prevention now recommends that IGRAs be the preferred test for patients with prior BCG vaccinations and those who are unlikely to return for a TST reading (e.g. homeless persons). But why not go one step further and abandon the tuberculin skin test entirely?

In an editorial in this week's JAMA, Drs. Phillip LoBue and Kenneth Castro examine the case for replacing traditional skin with blood tests for tuberculosis. They note that although the evidence is limited, positive IGRAs are able to predict which patients will eventually develop active tuberculosis in the absence of treatment at a rate similar to TSTs, albeit poorly (5 to 10 percent). It is also uncertain if having a single positive cutoff value for IGRAs will produce additional false-positive results, compared to the 3 values for a positive TST based on a person's health status and tuberculosis risk. Finally, an IGRA costs 3 times as much as TST, even factoring in costs associated with a follow-up visit.

What test or tests does your practice currently use to screen patients for latent tuberculosis infections? Do you think the convenience and ease of interpretation of the blood test is worth the additional cost? Please let us know your thoughts.