By Michael E. Zegans, MD
This small study is among the first to report on the use of interferon gamma release assays (IGRA) in U.S. uveitis patients. The study's authors reviewed the charts of patients seen at one ophthalmic practice over a three-year period for whom the IGRA blood test, QuantiFERON-TB Gold (QFT-G), was ordered.
The diagnosis of ocular tuberculosis (TB) is often challenging in both high and low incidence settings. In recent years, new TB testing methodologies, such as IGRA, have been developed. IGRAs assess the release of interferon gamma from a patient's T cells, which indicates the presence of previously TB-sensitized cells.
Four individuals (15 percent) in the current study were deemed to be true positive TB cases and had ophthalmic manifestations of ocular TB. Results of 13 tests (48 percent) were negative and six (22 percent) were indeterminate. Two patients (7 percent) had positive test results but, when subsequently retested, received negative results. The authors noted that the high rate of indeterminate tests was reduced when their protocol for collecting the samples was changed and different collection tubes were used. The complicated results suggest that IGRA testing still requires careful interpretation.
Retesting of positive individuals may be important to confirm results. It is imperative to recall that, as with skin tests, patients with active TB may have a negative IGRA test. This seemingly paradoxical finding is the result of the fact that patients may progress from latent to active TB at times when their immune response to TB is diminished. Since both the tuberculin skin testing and IRGAs assess the immune response to TB, both may be falsely negative when a patient has active TB.
IGRAs have been more sensitive and specific than traditional purified protein derivative (PPD) testing in the detection of TB infections in nonocular TB patients, particularly in distinguishing latent TB from previous bacille Calmette-Guérin (BCG) vaccination. IGRAs also may have advantages in immunosuppressed patients. Finally, IGRA testing has the benefit of requiring only one patient visit and not inducing a booster effect, as can occur in skin testing. The ease of testing is particularly attractive to ophthalmologists, who normally would have little occasion to place and read skin tests. U.S. ophthalmologists probably need to start becoming aware of these tests and their potential benefits and limitations.
Eye, epublished May 2, 2008
Financial Disclosures
Dr. Zegans receives grant support from the National Eye Institute.