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To the Editor:
Per recommendations from the Centers for Disease Control and Prevention, interferon g release assays (IGRAs) are being used increasingly in place of the tuberculin skin test (TST) for annual screening among US health care workers (1). Early adopters of the QuantiFERON-TB Gold In-Tube assay (QFT; Qiagen, Carnegie, Australia) are now reporting higher conversion rates compared with TST (2). In the largest health care worker study conducted to date, investigators reported six to nine times higher conversion rates with IGRAs compared with TST (3). Given the growing use of IGRAs, it is essential to identify and address the underlying sources of false conversions.
IGRAs are functional assays that are susceptible to modulation by endogenous and exogenous factors. Exogenous sources of variability identified thus far include variable blood volume in the QFT TB antigen tube, variable vigor of tube shaking, and processing delay (2). Endogenous immunomodulators originating from the host remain unknown. It is well known that blood samples obtained through venipuncture could harbor skin microbiota (4, 5). Microbe-associated molecular patterns are potent modulators of innate and adaptive immunity (6). Microbe-associated molecular patterns have also been shown to modulate the function of effector T cells (7, 8).
In a prospective study approved by the Stanford University Institutional Review Board, we assessed the effect of microbial contaminants on the specificity of the QFT assay. Uninfected volunteers were recruited from the clinical laboratories at our institution. Inclusion criteria included a prior negative TST and/or QFT and no tuberculosis (TB) risk factors. To investigate the effect of skin microbiota and environmental contaminants introduced into the QFT tubes during phlebotomy, we spiked nil and TB antigen tubes with 10 ml containing 0, 10, and 1,000 colony- forming units (CFU) Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa and assessed TB response (TB antigen minus nil) in 58 uninfected volunteers. S. aureus was representative of skin microbiota (4, 9), whereas the other two bacteria were representatives of environmental contaminants. For inoculum preparation, fresh colonies from sheep blood agar were suspended in endotoxin-free saline to an optical density A600 of 0.5 and passaged through a 5-mm filter to create a single-celled suspension at z1 3 107 CFU/ml. Phlebotomy, inoculation of QFT tubes with 1 mL blood, incubation and processing, ELISA, and...