Use of an interferon-gamma release assay to diagnose latent tuberculosis infection in foreign-born patients.

BACKGROUND

The tuberculin skin test (TST) has a low specificity in the setting of bacille Calmette-Guérin (BCG) vaccination. Interferon-gamma release assays (IGRAs) appear to be more specific but have not been validated in this population under routine clinical conditions. We sought to validate the routine clinical use of the T-SPOT.TB test (Oxford Immunotec; Oxford, UK), an IGRA, in a predominantly foreign-born population with a high rate of BCG vaccination.

METHODS

We compared the TST and the T-SPOT.TB test in 96 subjects at a New York City Department of Health tuberculosis clinic. We aimed to determine which test better predicted being a close contact of a case of active tuberculosis, a surrogate for latent tuberculosis infection.

RESULTS

A positive T-SPOT.TB test result was strongly associated with being a close contact of a case of active tuberculosis after adjustment for potential confounders (adjusted odds ratio, 2.9; 95% confidence interval, 1.1 to 7.3; p = 0.03). A positive TST result was associated with being a contact only in subjects without BCG vaccination (p = 0.02). The T-SPOT.TB test was more specific for being a close contact than the TST (p < 0.001). Specificity in BCG-vaccinated subjects was 3% for the TST compared with 70% for the T-SPOT.TB test (p < 0.001).

CONCLUSIONS

The T-SPOT.TB test is superior in routine clinical use to the TST for identifying high-risk individuals among foreign-born populations with high rates of BCG vaccination.