Predictive factors for the indeterminate result on the QuantiFERON-TB Gold In-Tube test

Background: QuantiFERON-TB Gold In-Tube (QFT-G IT) test is recommended for screening of latent tuberculosis. However, this can reveal the indeterminate results, especially in the immunocompromised patients. Therefore, we investigated the predictive factors that can influence the indeterminate results on QFT-G IT. Methods: We performed a retrospective case-control study with 144 cases with the indeterminate results on QFT-G IT from October 2006 to March 2009 at Severance Hospital. The controls with the positive or negative results were selected by matching 1:1 with the case for date of QFTG IT assay. An indeterminate result was considered when the level in both the tuberculosis antigen and mitogen tube was less than 0.35 IU/ml, or the level in the nil tube was higher than 8.0 IU/ml. Through retrospective review of medical records, age, gender, total lymphocyte count at the QFT-G IT assay, underlying diseases, and medical conditions which can influence the immune function were collected as covariates. The predictive factors for the indeterminate results on QFT-G IT were assessed by multivariate logistic regression models. Results: Age and gender did not show the significant differences between case and control patients (p = 0.105 and 0.906). The percentage of patients with hematologic malignancy (19.4% vs. 11.1%, p = 0.049), chronic renal disease (22.2% vs. 9.7%, p = 0.004), and autoimmune disease (16.0% vs. 6.3%, p = 0.009) in the case group were the significantly higher than those in the control group. Also, the case group had the significantly higher percentage of steroid use (16.0% vs. 8.3%, p = 0.047) and solid organ transplantation (11.1% vs. 2.1%, p = 0.002) than the control group. The total lymphocyte count was the significantly lower in the case group than in the control group (median [interquartile range], 540 [220—1,030] vs. 1,370 [870—1,970], p < 0.001). In the final regression model, clinical factors associated with the indeterminate results were chronic renal disease (odds ratio [OR] 2.26, 95% confidential interval [CI] 1.03—4.94, p = 0.041), autoimmune disease (OR 3.99, 95% CI 1.65—9.66, p = 0.002), and solid organ transplantation (OR 5.06, 95% CI 1.30—19.73, p = 0.020). Conclusion: The patients with lower lymphocyte counts or immunosuppressive condition showed the higher possibility of the indeterminate results on QFT-G IT assay. Therefore, the combined assay such as tuberculin skin test and T-SOPT TB with QFT-G IT may be helpful to screen the latent tuberculosis in the patients with these conditions.