QuantiFERON-TB predicts tuberculin skin test boosting in U.S. foreign-born

Santa Clara County, Northern California.

To characterize agreement of tuberculin skin test (TST) and QuantiFERON-TB (QFT) with repeated testing.

Fifty-two subjects participating in an ongoing prospective study of infectious disease transmission were tested by TST and QFT at two home visits 3 months apart. Boosting was defined as reclassification of TST from negative to positive. Agreement and reproducibility of TST and QFT were assessed using kappa and McNemar statistics.

Of 48 individuals completing all tests, 75% were foreign-born (92% Latin America) and 58% were BCG-vaccinated. Initial TST and QFT were positive in 13 (27%) and 21 (44%), respectively, with an overall agreement of 67% (K = 0.29). Ten (29%) of 35 initial TST-negative reactions boosted, nine of whom were BCG-vaccinated subjects. Boosting occurred in eight (67%) of 12 subjects who were initially QFT-positive/TST-negative. Compared to the second TST, initial QFT had a relative post-test probability of 76% (95% CI 0.58-0.95); boosting accounted for 8/16 (50%) of initial testing discordances.

Positive QFT in the setting of negative TST frequently anticipates a TST boost. This finding helps explain discordance between the two tests and may provide an alternative to serial TST testing.