Setting: The QuantiFERON®-TB Gold (QFT) assay is an interferon-gamma release assay used for the clinical diagnosis of latent tuberculous infection. Relatively high rates of indeterminate results are a significant downside of the test.
Objective: To evaluate clinical variables associated with lower QFT-indeterminate rates after reducing pre-incubation delay.
Design: In 2016, a new protocol of on-site incubation of QFT samples, followed by analysis at the outside laboratory, was implemented, resulting in much shorter pre-incubation delay of the samples. We retrospectively identified 3583 patients who underwent QFT before and after implementation of the new protocol. Patient records were scrutinized and QFT results were evaluated with respect to associated clinical conditions.
Results: The patients were analyzed by dividing them into two cohorts based on maximum pre-incubation time (standard vs. immediate). Monthly indeterminate results dropped from 12.7% ± 0.02 in the standard cohort to 5.5% ± 0.03 in the immediate cohort (P < 0.001). A significant reduction in relative indeterminate rates was found in the immediate cohort patients with immunocompromised state, autoimmune conditions, liver disease and hypoalbuminemia. No difference was identified in patients with malignancies and renal failure.
Conclusion: Limiting the pre-incubation time to 1 h maximum significantly improved QFT performance, especially in patients from certain clinical categories.