It is important to identify and treat persons with LTBI to prevent progression to active disease.
The tuberculin skin test (TST) is traditionally used for the identification of prior exposure to Mycobacterium tuberculosis. However, specificity is reduced if an individual has had the vaccination Bacille Calmette-Guerin (BCG), which is intended to prevent TB, or has been infected with nontuberculous mycobacterium (environmental mycobacterial species).
In these cases, blood tests, collectively known as interferon-gamma release assays (IGRA), may be more effective in identifying LTBI. IGRAs are more sensitive and specific, do not require the individual to return for test interpretation, and are not subject to possible inaccuracies and bias in reading the test, as may be the case with the TST. The turnaround time is usually less than 24 hours. However, the blood sample must be processed within 8-30 hours after collection while white blood cells are still viable. IGRA tests are more expensive to perform than the TST and not as widely available. There is also limited data on the ability of IGRAs to predict the risk for progressing to tuberculosis disease.