789. Comparison of Interferon-γ Release Assays (IGRAs) for Diagnosis of Latent or Active Tuberculosis in Cancer Patients

Background Patients with certain types of cancer are at increased risk for progression from latent tuberculosis infection (LTBI) to active tuberculosis (ATB) because of immunosuppression. The purpose of this study was to compare the utility of the two commonly used IGRAs, QuantiFERON-TB Gold® (QFT) and T-spot.TB® (T-spot.TB), for diagnosis of LTBI or ATB in cancer patients. Methods We identified patients who had an initial IGRA during 2013 and 2014 at our institution. Along with demographic information, collected clinical data included type of underlying cancer or other condition, reason for testing, diagnosis of ATB following testing, and absolute lymphocyte count (ALC) at the time of testing. IGRA results (positive, negative, borderline, or indeterminate/invalid) were compared between patients who underwent testing with either QFT or T-spot.TB. Results A total of 356 patients had 411 QFT tests done, while 737 patients had 853 T-spot.TB tests performed. The most common underlying malignancies in the QFT and T-spot.TB groups were acute myeloid leukemia (30% and 25%, respectively) and solid tumors (28% vs. 30%, respectively). The most common reasons for testing were pre-hematopoietic-cell transplantation (HCT) screening (42% with QFT and 31% with T spot.TB) or suspected pulmonary ATB (34% with QFT and 42% with T spot.TB). In the QFT group, 145/411 (35%) tests were indeterminate, while only 96/853 (11%) tests in the T-spot.TB group were invalid (P <0.001). The median ALC was 650 cells/µL in patients with an indeterminate result in the QFT group and 90 cells/µL in patients with an invalid test in the T-spot.TB group. A total of four patients were diagnosed with ATB at 1 year after testing. Figure 1 provides a flowchart describing IGRA testing results and development of ATB. Conclusion The frequency of an inconclusive test result is significantly higher with QFT as compared with T-spot.TB for diagnosis of LTBI or ATB in cancer patients. A low ALC is likely a contributing factor in indeterminate QFT and invalid T spot.TBresults. Disclosures E. Ariza-Heredia, Oxford Immunotec: Grant Investigator, Research grant. R. F. Chemaly, Oxford Immunotec: Consultant and Grant Investigator, Research grant.