Screening for Mycobacterium tuberculosis using an interferon-gamma release assay

The Centers for Disease Control and Prevention recommend analyzing characteristics of populations screened for Mycobacterium tuberculosis infection using interferon-gamma release assays (IGRAs).

We characterized requests for IGRA analyses submitted to the first laboratory in Washington State that began to offer IGRAs as a tuberculosis screening test. In addition, we chronicled the process by which this program was launched.

Through a retrospective record review we recorded the IGRA results, source of the request, and reason(s) for each request along with demographic information, risk factors, and prior exposure for each subject. Through dissemination and quality control measures, IGRA analyses began within the health district but quickly spread throughout the state and the West.

A regional health district clinical laboratory in the Pacific Northwest received 4422 IGRA requests for 4218 unique subjects from January 2, 2008, through June 5, 2009.

Most (96%) requests originated from public health clinics or centers, hospitals, private laboratories or practitioners, and Indian Health Centers. Community and migrant health centers had the highest rates of positive IGRA results (28%, Φc = 0.159, P = .000). About one-half of all requests were for employment or high-risk surveillance. Persons with a positive IGRA had a larger TST induration (M = 16.66 mm, 95% CI: 15.18-18.14) than those with a negative IGRA (M = 13.53 mm, 95% CI: 12.92-14.15) but did not differ for those with an indeterminate IGRA (M = 13.29 mm, 95% CI: 5.95-20.62).

Public and private agencies use IGRAs for surveillance of persons employed, routinely screened, at high risk, or foreign-born. The TST induration varies widely for persons with an indeterminate IGRA. The process for implementing IGRA analyses serves as a model for other programs seeking to conduct similar reviews of test use and results.