Utility of the T-SPOT ® .TB test's borderline category to increase test resolution for results around the cut-off point.

BACKGROUND: Accurate identification of individuals with TB infection, is required to achieve the WHO's End TB Strategy goals. While there is general acceptance that the T-SPOT.TB test borderline category provides an opportunity to increase test resolution of results around the test cut-off point, this has not been investigated.

METHODS: 645,947 tests were analyzed to determine frequency of borderline results, effect of age and time between tests and associations between subjects' clinical risk factors and retest results.

RESULTS: 645,947 tests produced 93.5% negatives, 4% positives, 0.6% invalids, and 1.8% borderlines. Within the borderline results, 5044 were repeated, with 59.2%, 20.0% and 20.2% resolving to negative, positive and borderline, respectively. Age of subject did not affect retest results; however, time between tests indicated that retest resolution occurred with greatest frequency after 90 days. TB risk factors were provided for 2640 subjects and 17% of low risk subjects with a high initial borderline resolved to negative while 27.6% of subjects with high risk and an initial low borderline resolved to positive, suggesting that these subjects could have been inappropriately classified if using a single cut-off point test with no borderline category.

CONCLUSION: This study demonstrates the utility of the T-SPOT.TB test's borderline category to increase test resolution around the test cut-off point.