Comparison of an interferon-γ release assay with tuberculin skin testing in HIV-infected individuals

Rationale: Although interferon (IFN)-γ release assays are approved for the diagnosis of latent tuberculosis infection (LTBI), limited data exist regarding their performance in HIV infection. Objectives: To compare tuberculin skin test (TST) results to the commercial IFN-γ release assay QuantiFERON-TB Gold In-Tube (QFT) for the diagnosis of LTBI in HIV-infected adults. Methods: A total of 294 HIV-infected subjects sampled from two San Francisco cohorts underwent TST, using 5 TU of purified protein derivative, and QFT, measuring IFN-γ response to Mycobacterium tuberculosis-specific RD-1 antigens. Main Results: Of 294 participants, 205 (70%) returned for an evaluable TST. Concordance between QFT and TST was 89.3% (kappa = 0.37, p = 0.007). However, in subjects with positive test results by either TST or QFT, only 28% (8/29) had positive test results by both modalities. TST-positive/QFT-negative discordant results were found in 5.1% of subjects and TST-negative/QFT-positive discordance in 5.6%. Indeterminate QFT results occurred in 5.1%, all due to a failure to respond to the phytohemagglutinin-positive control. Subjects with a CD4⁺ count of less than 100 cells/mm³ had a relative risk of an indeterminate result of 4.24 (95% confidence interval, 1.55-11.61; p= 0.003) compared with those with a CD4⁺ count of 100 or more. Conclusions: Overall concordance between QFT and TST in HIV infection was high, but agreement among subjects with positive tests by either modality was low.