Comparison of Predictors of Indeterminate QuantiFERON-TB Gold In-Tube Test Results between Inpatients and Outpatients

Background:

Indeterminate QuantiFERON-TB Gold In-Tube (QFT-GIT) test results pose a challenge for clinical management. Goal of this study was to compare the rates of indeterminate results in ambulatory and hospitalized patients and to identify clinical factors associated with indeterminate results.

Methods:

We evaluated all inpatients and ambulatory patients who received QFT-GIT testing during either hospitalization or a clinic visit between 1/1/2012 and 9/30/2015. Demographic data, clinical, laboratory factors were evaluated, and their associations with indeterminate QFT-GIT results were compared between the two cohorts.

Results:

756 patients were studied, 426 inpatients and 330 ambulatory patients. 155/426 (36.4%) of test results were reported as indeterminate in the inpatient group, whereas only 8/330 (2.4%) indeterminate results were found in the outpatient cohort (p<0.001). In the inpatient setting, poor performance status (PS≥2), history of ICU stay, abnormal WBC count (<4 or >10 x 10⁹/L), lymphopenia (< 1 x 10⁹/L), anemia (Hb ≤12 g/dl), hypoalbuminemia (≤3.5 g/dl), low serum total protein (<6 g/dl) were significantly associated with indeterminate results in bivariate analysis. In the multivariate model, ICU stay (p=0.011), absolute lymphocyte count (p<0.001), Hb (p=0.005), albumin level (p<0.009) were independent predictors. In the outpatient setting, HIV positivity (p=0.002), use of immunosuppressive medications (p=0.008), anemia (p=0.005), and low total protein (p=0.012) were associated with indeterminate results in bivariate analysis, with HIV status (p=0.016) and anemia (p=0.012) retaining their independent significance.

Conclusion:

Inpatients and ambulatory patients differ in the rate of indeterminate QFT-GIT results and in associated risk predictors of an indeterminate result. Critical illness, hypoalbuminemia, low lymphocyte count appear to be important factors in inpatients, whereas HIV positivity is an independent predictor for ambulatory patients. Anemia is encountered in both settings as an independent risk factor. Knowledge of the above clinical characteristics may help providers in utilization of QFT-GIT and may help reduce testing in hospitalized patients likely to have indeterminate results.