IDO1 and tuberculosis: Active TB patients have increased activity of serum IDO [22], however, we saw no difference in IDO activity by LTBI status, indicating that IFN-γ secretion in response to ESAT-6 and CFP-10 (used here to define LTBI) does not necessarily reflect chronic exposure to MTB or that the exposure is below the threshold required to see a change in systemic IDO activity.