Indeterminate QuantiFERON^® -TB Gold results in a public health clinic setting

SETTING: Public chest clinics, New York City. OBJECTIVE: To evaluate the association between patient characteristics and indeterminate QuantiFERON ® TB Gold (QFT-G) results and describe follow-up testing.

DESIGN: Retrospective cross-sectional multivariate logistic regression analyses of patients receiving QFT-G from October 2006 to June 2008. Analyses were performed separately depending on whether the indeterminate result was due to positive (low mitogen) or negative (high nil) control failure.

RESULTS: Of 28 864 individuals tested, 2058 (7%) tested positive, 26 284 (91%) negative, and 522 (2%) were indeterminate (low mitogen, n = 264; high nil, n = 258). Low mitogen results were more frequent among those aged < 10 years (OR_adj 3.7, 95%CI 2.4–5.9), females (OR_adj 1.4, 95%CI 1.1–1.8), Asians (OR_adj 2.1, 95%CI 1.3–3.4) and the US-born (OR_adj 1.9, 95%CI 1.4–2.6) and less frequent among Hispanics (OR_adj 0.6, 95%CI 0.4–1.0). High nil results were more frequent among Hispanics (OR_adj 1.7, 95%CI 1.0–2.8) and less frequent among the US-born (OR_adj 0.6, 95%CI 0.5– 0.8). Among patients who received a repeat QFT-G (n = 137), 4% tested positive, 64% negative and 32% had a second indeterminate result, always of the same type.

CONCLUSION: Age, race/ethnicity and sex were associ- ated with indeterminate QFT-G results. Most follow-up tests were negative, although a modest proportion were repeatedly indeterminate. Further studies evaluating the mechanisms of QFT-G indeterminate results are needed.